If the soft synthe's offer enough on the fly features to warrant controller usage then it would be worth it like eg. multiple LFO control / detunes / Osc vi and rez & q (esp. muti.) then heck yeah. I think most vst's arent that elaborate though. Personally, I am a control phreak when it comes to sound, and I like analog synthe's best. I would take a knob(1st) or a slider(2nd) over a button any day. esp a software button.
Wait for someone elses entry as it might be better cawse I am no master at soft synthe's. If anything, I'm 3rd seat bench on the farm team without my knobs to doodle with.
MORPH!
PS As always, I disclaim everything and this is just my opinion to not be held to.
Presume: I know nothing.