With the exception of Ozone, I use Sonar stuff exclusively.  I actually had a mini buying fever a couple days ago, had the credit card out, and was scouring the internet to buy some software/DAW thing, and I actually could not think (or justify) a single thing I came across, because Sonar has it covered. 
 
That being said, options are fun.  Not necessary, but fun.

I don't use very many third party effects. I think both the Sonitus Gate and the ProChannel gate are under-featured, so I use the free Reaper one. And I still use SPAN on everything, even despite the new Quad Curve analyser. That's fine for most things, but the SPAN hi res mode is really useful for tracking down problems really accurately.  There's not much else I'd miss significantly. Got some Brainworx plugins that are useful sometimes, but largely not critical.
In terms of synths, depends what you're trying to do, but there are a lot of them bundled, and I use most of them regularly.

I rely heavily on strummed acoustic guitars and that part is tough.  That may be possible with Dimension Pro and a lot of hand work, but I haven't tried it.
 
Otherwise, the bass, drums, rhodes, acoustic piano, etc. are plenty good enough for a record.  While the strings are not as good as a massive library like Vienna and may not be ideal when exposed or soloed, they would be fine in a mix--and not too many years ago I would have killed for string sounds that good.
 
Lots of hit records were made with a lot less.
 
As far as the effects and plug-ins go, I rely on the built-ins for most things and they are fine.

As far as effects go, IMHO Sonar X3 has everything covered. But I do love to use concrete limiter (70 USD from Cakewalk) and I also own Alloy 2 which I had rarely use.
 
But as far as softsynth goes I need a lot more than CW offers. I am a pianist composer so I got myself the Ivory 2, Truepiano and Addictive Keys.
 
It really depends on what type of music and sounds you are after. If your are into EDM you might want to into Zeta+2. Only a 50 USD upgrade if your are Sonar X1/2/3 owner. And you get an awesome interface plus 1000 new sounds compare to the zeta that came with Sonar X3 PE.

More is better, of course.  up to a point, anyway, and then you don't have the time to try out everything.  But at this stage of SONAR, Producer has everything you need.  A proper sampler would be nice, as noted above (tho I love DimPro, it is a sample based synth).  And it is funny, one of the weak spots I found w/ SONAr Pro was time based effects, like delays.  Sonitus was OK.  So I got the Nomad stuff years ago to use.  Now it is part of the X3.
 
@

Agree that right out of the box X3 Producer has everything neccessary for a high quality result. That said I think upgrading Melodyne to Editor is a worthwhile investment. I do have quite a bit of 3rd party stuff (mostly NI) and I use a small portion of the effects and several of the instruments from NI but to be honest I think the Pro Channel suite of tools are very high quality and satisfy most of my requirements quite nicely.
 
Regards,

If you want to hear some great recordings just using the plug ins that come with Sonar, go to the song forum and you will discover the magic that can be achieved with just Sonar alone.

In addition to external synths, you can see from my signature line that I like sample libraries.  That said, I ahve thought about installing only basic Sonar (and maybe RealGuitar) on my laptop for composing while traveling. 
 
I think Ivory 2, for example, is better than the bundled piano, but in a record I am not sure if anyone would even think about it.  The Scarbee basses (Jay Bass, P Bass, and Rick) are generally better than the SI-Bass, but I have some tracks where I like the SI-Bass better.  (The SI-Bass sounds like a P-Bass to me, but I have no idea what instrument was sampled.)
 

The demo song is a decent example of what can be achieved using just stock plugins. Many of our favorite songs of all genres were done with a lot less. Are there better plugins on the market? Yes ( and even that is very subjective).  The same way there are "better" studio setups with more toys.  Does not necessarily make for a better product.  I find myself using third party plugs less and less.  I refuse to buy another EQ unless someone shows me some serious numerical and audible examples of a whopping difference between third party EQ's and the ones bundled with Sonar. Compressors are likely to produce more variety, but the bundled ones more than cover just about every situation.  I think reverb may be the most compelling case for third party solutions and honestly, I could get by using nothing  the sonitus verb.  The rest are a bonus to me.  As already pointed out, the addition of the Nomad plugs greatly helps the delay area.  If you are still using 32-bit, the old DSPfx gives you additional older but quality delay effects.
 
As far as instruments, that is far more subjective and the answer to whether you can stay strictly with Sonar gets more complicated.  Even then, I would say most synth based music can be done but some genres would be harder than others.  The one truly missing element is a full featured sampler and even that is more of a nuisance than deal breaker.  For genres that are heavily sample based, Sonar has enough sample manipulation tools to pull off much of what samplers do.  If one is really on a budget, I don't think itis a must to upgrade to Z3ta+2.  Yes, it is better, but most of the sounds achievable with it can be done with classic version that is included.  The inclusion of Rapture (not to mention some older but still powerful synths) covers a lot of synth ground .  A surprising amount of pop music is done with presets, so if one is looking for that exact patch from something like Massive, you are out of luck.  The one weakspot for synths is not having a true FM powerhouse. As far as drums, if you can't create a decent drum track between Addictive drums, SD3 and the drum patches in there other synths, you probably just make crappy drum tracks and no plugin will change that.  That is not to knock other great programs or to say Sonar's way of making drums is best and most intuitive  for every genre.  Just saying it can be done.  These days I find myself more focused on mastering what I have instead of collecting a bunch of plugs . 

I wouldn't call this really GREAT, but it's something I did a couple years ago with X2 and z3ta2: https://soundcloud.com/mike-leghorn/trance-dance-of-the-lederhosen
 
I used one non-Sonar product, for reverb. Sonar doesn't come with a decent reverb, in my opinion.