Hi danny,
np :)
In the case of the OP.. the Waves C6 plug-in only supports sidechain functionality if you use the VST3 version..if you use the VST2.4 version it doesnt... So having spent a sizeable amount on the plug he cannot use it to its full potential in Sonar...The same thing applies to some other VST3 plugins that folks either have a license for and cannot use, or wish to buy but cannot due to the limitation in Sonar...
Similarly for hardware, there are editors that are only available in VST3 format.. for example all current Motif workstations and derivatives incl. my S70xs stage piano.. I have a VST3 editor that i cannot use and so integrating the huge capability of the S70xs into a Sonar project is simply not possible.. i have to dumb down my needs of it and use it as a simple sound generator.
So, to answer the question "what are the benefits? etc.... the answer is simply that the consumer can pick what plugins and hardware to use, rather than be constrained in choice by the DAW vendor... (I think you answered this yourself with "unless there are specific VST plugs that could not be used")..
cheers,
p.s. that a jackson?.. imagine being told you had to play heavy metal with a Tele :)