I never saw OS X animations stutter. Sure, Expose drops frames now and then (dropping frames is the sensible approach, not displaying every frame and introducing stutter), but that's ok. That makes the animation jerky but just as quick and reduces the impression of stutter.

I think it is very plausible that seamless and low key animations can increase usability. Sure, user testing would be needed to verify that, but it seems intuitive enough to me. There are technical problems, sure, and stuttering animations should be avoided, but that's no reason to reject morphing out of hand.