One final plug-in story.
 
Sound on Sound magazine does a feature called "Mix Rescue" where they go to a reader's house and assist with a mix. They arrived at their host's, who went to make tea for them. While he was doing that, they bypassed all the plug-ins to hear what the raw tracks sounded like.
 
When the host came back to the studio with the tea, he said "Wow! That sounds so much better!! What did you guys do?!?"

We had the exact same thing happen on the forum, someone came in and said "after working on my mix for a few hours I hit 'E' and suddenly everything sounded much better... What does 'E' do?" :)

Pro to Plat? I never really thought twice, never looked back. Splat with lifetime updates as well as platinum exclusives, well worth the price. Money well spent! 

I have Platinum and I only use the included plug-ins.  It is a hobby for me and I probably don't produce "pro" sounding stuff (but that is more likely due to talent, creativity and experience) than the tools.
 
The instruments are good and the FX are good (especially the Pro Channel).  Sure, you can spend more money and get better, but its more money than you'd spend on the upgrade to Platinum.
 
Again, lifetime updates are the icing on the proverbial cake.

Arizona, as to instruments...you get the full version of Rapture (not the newer Rapture Pro that also loads Dimension Pro files, the original one) with Platinum. This has been one of my absolute favorite instruments ever since it first appeared. I can't think of any track I've done over the past several years that doesn't use it somewhere, and several tracks have four or five instances. It was the first synth I found that could do extensive modulation, which made it perfect for EDM but it could also do straight ahead synth sounds - I posted a comparison online once of Arturia's Minimoog V playing the same thing as Rapture with big filter sweeps etc...no one could tell the difference.
 
You also get Dimension Pro which has a large library, and Rapture Session. To me the presets/samples are hit-and-miss. Some are useless, some have become go-to presets. Admittedly the fact that I use mostly my own samples and presets factors into that opinion.
 

Yes, the number one feature that makes PLATINUM worth buying is the current offer of lifetime updates.  This alone is worth the money.  I may go 2 or 3 years without updating the core software, but at least the option is there.
 
I still run win 7 pro.  I have an M-audio ProFire 2626 (firewire).  When I went to update to Windows 10, I got a rude awakening.  M-audio is not going to update the driver for the profire 2626.  The unit is good and functional, I rolled back to win 7 pro for that 1 piece of equipment. 
 
I bought a laptop with Windows 10 ugh.... Went to put Sonar X3 on it... it did NOT play nice.  Sometimes the hardware that you have and works great with what you already own for now but will not work correctly with newer, more modern equipment.  I can't update my hardware in the sense of downloading an update.  So I am happy that I can at least get lifetime updates, whether I need them or not, for my Sonar software.  Now if Cakewalk goes out of business next year, my "lifetime" updates will be useless.  Who knows if the next version of windows won't break sonar? 
 
The soft synth plugins of Platinum are just fine.  The main one that I use is Addictive Drums 2.  LOVE...LOVE....LOVE IT!  I do not do a lot of synth creativity, but when I do, I go to dimension pro A LOT.  I purchased Native Instruments KOMPLETE ULTIMATE and within a month, realized I dumped a lot of money on something that bogs down my system.  Does Komplete have great synth sounds? Sure.  Do I like waiting what seems like an eternity for any of the synths to load?  No, I don't.  DPro, Rapture and Z3ta load up just fine and play fine enough sounds for my sonic palate.  And these are not terribly resource hungry.
 
A good way to compare is which works better the ProChannel PC2A plugin or the Native Instruments VC2A plugin?  For me the sound so close that eq will take care of any difference.  So do I pop in PC2A in the prochannel and keep moving or do I bog my system down waiting for VC2A to verify I have a legal right to use it?  I'll go ProChannel for timeliness.
 
FX Plugins.  I've used Sonitus since forever.  Remember the ad in the movie "Crazy People" about Volvo... "They're boxy, but da.. good"  Same thing here.... "Sonitus, They're ugly, but da.. good."  I find myself using the Blues Tubes plugins more than the prochannel modules.  I liked the blues tubes so much that I found Nomad Factory's ISP 3 suite on sale and snatched up all of the nomad factory plugs.  The BREVERB plugin...I liked it so much that I bought the full blown version.  I don't have any complaints about the quality of the ProChannel modules.  They are good,  I just get lost in the collapsing and expanding of channelstrips.  When I lose my place two or three times, and I realize I've adjusted the wrong plugins two or three different times, I realize that it is just easier for me to load individual plugins and click on the plug when I need to adjust it. 

Regarding Platinum's included plug-in instruments, I would add that having the full Dimension Pro and Rapture classic is a big plus.  Dimension Pro has a large sample library, and offers a lot of sound programming tools.

As Craig said earlier, Rapture is an excellent sounding synth with some very deep programming tools.  It also comes with a ton of presets. 
 
Both of these synths are great if you like to look under the hood and get your hands dirty.

True Pianos Amber is a great sounding grand piano. 

Then there are also the bundled AAS instuments:
AAS Lounge Lizard Session - Sonar Edition, with some great modelled Rhodes piano sounds.  
AAS Strum Session - Sonar Edition, which models acoustic and electric guitars, as well as chord voicing and strum patterns.
AAS Ultra Analog Session - Sonar Edition, a modelled VA synth.

Then when you consider that you also get the full Pro Channel modules and additional FX in the boutique suite, you really get a lot more content with Platinum.  I really like BREVERB Sonar and the QuadCurve EQ flyout
 
Overall I don't use all of the tools in Platinum, but I don't think I would want to give them up.  It's nice to know they are there if I ever need them.  It's a great core collection to start out with, before spending more money on 3rd party plugins 

In a way, but the question is is it worth to spring for Platinum or better to save money and go third party.

One thing that is often overlooked is how are we defining "sounds better" when comparing instruments?
 
How is a fair comparison to be made?  The bottom line is the perception and intended use by the end user.

Are we judging the samplers and synths in the same way?  Based on what? Sample quality, factory presets, sound source (sample playback, analog modeling, physical modeling), the sound architecture, the available expansion packs, the ease or depth of programmability?

Some folks just want preset sounds to quickly set up an arrangement.  That's fine.  Others may want to develop their own sounds and presets.  So they want more capability under the hood.   Different styles of music have varying sonic expectations.  I doubt there is one instrument that meets everybody's needs.  Just look at the sheer number of the available plugins and libraries.  None of them sound inherently bad.  But there are good and bad patches everywhere.
 
That said, I like Dimension Pro, but I wouldn't buy Platinum just for that.  I don't think that Dimension was ever intended to compete with the other commercial sampler libraries, but was intended to provide a starting point for creative sound design.  It came with a library as a bonus.  It is unique.  It's a synthesizer that uses samples for a sound source.

But the synth technology that RGC audio (Rene Ceballos) developed, which was bought by Cakewalk, is a good one. It underlies all of the major Cakewalk synths - Triangle, Square, Pentagon, Dimension Pro, Rapture, Z3TA+ 2, etc.  The original RGC audio z3ta+ is still considered great by many.  I think Rene was possibly way ahead of the current pack of synth developers.
 

I guess I did a lackluster job of explaining "worth" and "value" versus "speed" and "simplicity"  I find value in the complete package of Platinum.  Can I go elsewhere sure?  I'm like Craig, I like the Native stuff just fine but when I'm on a roll, I want to move quickly and keep the flow going.  I could probably buy the Artist Version and do everything I want to do. 
 
But lets look at value
Artist $105                Pro:  $209         Platinum:     $499
 
If I have to buy Artist every three or for years, I will pay 300 to 400 dollars... it just depends on updates.
Pro, if I update twice, there's over $400 dollars.
 
Do I find value in the lifetime updates?  I sure do.  A windows update could break sonar.  Access to the update that fixes the functionality--- Priceless.
 
The extra goodies? It may be a big fat question that only your own soul will answer.  Upgrade pricing helps some.  Starting at the bottom of the rung and working your way up financially may also be beneficial.  I didn't start of Pro, I started with Music Creator #2 and tinkered until I decided if I wanted to take the next step.  My next step was Cakewalk Sonar X1 pro... you don't have to dive in to the deep end, take it slow... learn along the way.

This thread has been very helpful. Thanks to everyone for your comments and advice.