I think you'd be surprised Donny by how many pro studios are supplementing or substituting live musicians for synth and vst replications.

Back in the 80's, I was observing a Sears jingle session at Universal studios in Chicago with composer John Tagenhurst (ET composer w/John Williams). They recorded the string parts with an Emulator II.

With the advent of Genos, the sounds are getting pretty realistic. Especially, in the hands of a good player.

But, even if your doing demos...being able to tweak the mix once vocals are added can be a big plus. How many times do we listen to demos where the drums could be boosted...the lead guitar could use more verb, or the bass is to boomy. This would allow an easy fix.