||Here's my opinion on 2 points.
1) The spacers are actually more forgiving than full metal to metal attachments due to still having rubber to isolate certain vibration frequencies.
Subframe spacers give you more response from the rear subframe, yet still isolate some of the vibes compared to metal to metal.
3) The Honda with the metal to metal subframe attachments may not be the whole story as it seems. That car may utilize quite a few other rubber bushings between the suspension and it's final attachment to the subframe. In other words, suspension attachments may be so isolated from vibes at other points, that the engineers decided further isolating the subframe from the unibody was not needed due to that type of design. The Z's design may have lead to many more vibes going into the subframe, that the engineers decided to cure it by the isolating the subframe from the unibody with the subframe rubber/gel filled isolators.