rmfegley
Yeah, Steely Dan didn't need no fancy pants plugins. They made do with hundreds of thousands of dollars worth of state of the art hardware, the most talented and experienced musicians and engineers in the business and virtually limitless studio time. 
First of all, this was hilarious and spot on... I can't verify this independently but I've heard it said, and it seems plausible, that Aja was the first album that cost more than $1million to make. You can buy a *lot* of plugins for that.
Second, yeah, this discussion comes up frequently and many cogent things have been said on the subject, and there are a lot of factors. Certainly it's entirely possible to do nearly anything you can imagine in the way of 'normal' recording/producing/mixing/mastering tasks entirely within the confines of what Sonar, particularly in its Platinum flavour provides. If you know your way around the tools you can make it sound pretty gosh-darn good, too, without adding any outside plugins.
However... if I may... let's use a metaphor here: producing music is a lot like cooking. There are a lot of different ways of cooking in the world, and a lot of different styles of food, and a lot of different flavours and spices and specialized ingredients. There are also a lot of excellent chefs, and they all have different tools, techniques, special or favourite ingredients and spices that they use to do what they do. There is no *right* way of cooking, generally (though there are often numerous *wrong* ways, but even there - it's sometimes possible to get great results by doing things wrong, whether on purpose or in the 'happy accident' sense), and no *right* tool for any given job.
A great chef will likely be able to cook a very good meal with nearly any set of tools or ingredients, but in his or her own kitchen with all the the tools, spices, ingredients etc. that s/he is accustomed to, you're more likely to get a *great* meal. That isn't to say that those specific tools/ingredients/spices (whatever, the metaphor is getting stretched thin at this point) are absolutely required for any chef to make a great meal... you get the idea.
It also doesn't mean that anyone with that set of tools/ingredients will be able to create a great meal. It *definitely* doesn't mean that you need ALL the special tools and ingredients and spices that ALL the great chefs swear by - they certainly don't have all of them, and wouldn't use them all if they did. They have and use the ones they know will get them the results they want in the way they are accustomed to working, fast and reliably.
So here's the drill. The marketing departments of Waves, FabFilter, SoundToys, iZotope, Native Instruments, IK Multimedia, and so on, are paid to make you want the products those companies make. The products may or may not be excellent - I've got stuff from all of them, and most I think are great, and some I even use a lot. I've been stuck in GAS loops from time to time (Gear Acquisition Syndrome, for any unfamiliar with the term) and sometimes I end up with something that is really inspiring and useful, other times I spend a bunch of money for something that seems awesome and indispensible, but I never end up using (but someone else might use on every track). So it goes.
So... isn't it great that Sonar (and every other major DAW including, finally, unbelievably, Reason) gives you the option? It's like an amazing, fully-appointed high-end kitchen with a solid set of tools - some of which, like the above-mentioned Adaptive limiter, are on par with just about anything out there - that any decent chef can use to get great results, *but* it's also possible to bring in whatever else you think is essential to working however you want.
For example, I have a penchant for kind of wacko, out-there experimental sounds and processing, for which I would have to stretch the tools in Sonar pretty hard to get close to but which something like NI's Molekular or Melda's MGranularMB make easy and fun to experiment with. I like RapturePro very much but as flexible and powerful as it is, I also have a raft of synths from NI, Arturia and others to play with, and while I could certainly make music without them I wouldn't choose to - sometimes I fire up Iris, or Razor, or Loom, my new plaything MPowerSynth, and hit some randomizer buttons until something sparks and I'm off to the races. That's my way of working, but might not work at all for someone else, but they might feel that mixing without Slate's VCC is *impossible*, whereas I have no need for it whatsoever.
So there you go. If you're relatively new to cooking - err, music production, I'd say Don't Believe The Hype, stick with the great set of tools in Sonar and get used to them, there's a whole lot there and a whole lot to learn. You don't *need* anything else, and it will arguably only distract you. But past a certain point, sometimes a new flavour or spice or very cool tool will inspire you and open a door to a sound or a way of working you wouldn't have found without it, and that's how we grow.