I think you're off base a bit here. The kind of coupling you get on a solid surface (like a concrete slab) is acoustic, not mechanical, so vibration isolation doesn't do anything for or against it... since you can't significantly vibrate a concrete slab... with a bass rig anyway.
So with a Gramma, what you're doing is isolating a hollow stage from (at least hopefully a good amount of) the mechanical coupling, and still preserving the acoustic coupling. That's the ideal solution. Studio folks have been doing this kind of thing for YEARS with control room monitors. This is a WELL proven model.
However, I can see a downsides to trying to build vibration isolation into a cab. First of all, which side do you put it on? Sometimes we like to put our cabs upright, sometimes sideways. Also, you really want the cabinet to be free to vibrate and resonate. That's part of the cabinet design, so you'd be relegated into some really strange designs like having four connection points and air space between the cabinet wall and the vibration damper. Secondly, it adds cost to the cabinet, and (just as you point out above) vibration isolation is not always needed.
Considering the fact that stuff like the Gramma already exists out there, that it's not at all expensive, and the flexibility of having it be outboard gear you can use any way you like, that really seems like, by far, the best solution... what we already have.
