At a high level user vuew of functionality, then VST3 isnt really that big a deal, however it was needed as much of an architectural enhancement to deal with a bunch of pressing system issues - 64 bit, easier multi-channel scaling for higher end users. I dont buy that its is essential for side chaining - it isnt, however it does perhaps simplify this a bit at an architectural level.
For plugin developers however an architectural change can be a real pain, or not,it depends upon how you have designed your plugin. Multi-pltform plugin developers tend to fair better as they have often allready designed their plugins for easy adaptation.
As a developer starting to write a plugin - then actually Im targettig
VST 2.4 only - the reason is simple - I dont care about running it in Cubase 4, I will only ever been running it in Ableton Live 7 onwards. Actually If Ableton had a public plugin SDK for thier own plugins, I would use that in preference to
VST as it appears to allready address the benefits that VST3 could provide to a stereo DAW, and arguably in a better way for end users.