Is there any reason to keep VST/VST2 plugins in Sonar Platinum if there is a VST3 version of the same?
 
Thanks.

You will need them if they were used in any projects and the plug-in manufacturer does not support automatic VST2 to VST3 migration. Note: most manufacturers do not support it.

Some softsynths with vst2 and vst3 versions have vst2 versions that respond to program changes whereas the vst3 versions do not.  Meldaproduction's Mpowersynth is an example of such a synth.

There are also times when VST 3 versions of certain plugins may have some quirky behavior or even crash, whereas their VST 2 versions do not.  Arturia had some of that with some of their V Collection synths a while back.
 
Sonar will hide the VST 2 versions, by default, if there is a VST 3 version available.  Also, these modules aren't terribly big, I have 1200+ plugins and they don't take up all that much hard drive space.
 
Bob Bone
 

Maybe I'm just contrarian by nature, but I don't even install VST3 versions whenever it's an option.

Interesting, I don't install VST2 versions if there is a VST3 version.

Why?

i install both, if there is a vst3 i use that every time

Some plugins have VST3 versions that don't show the presets, if that matters to you.

bitflipper
Why?

Why not is there really any difference ?