The simple answer is that no riders of mortal skill levels ever seem to notice any difference between any of the possible set-ups. I'm open to input from people with talent who might be able to feel/appreciate the difference.
My understanding of the theories:
1. A floating rear brake should isolate the rear brake from the suspension movement. Its interesting to note that Yamaha used a floating rear brake on some of their larger capacity trail bikes (DT1, DT360A etc), but didn't bother with them on the YZs until the long travel 1979 models - I'm sure that there's a lesson in that - but being Yamaha, you can't tell how much of it was for real reasons, and how much was about marketing/gimmickry...
2. A cable rear brake is about packaging more than anything, AFAIK. A cable makes cross-over (pedal to brake) arrangment a lot easier, and also removes the need for the pedal-rod pivot to be at/near the swing arm pivot. Cables also seem to take a bit of the 'edge' off the brake operation, which would probably be a good thing for a lot of bikes (ie: old Yamahas)...
Its probably worth noting that cable operation, and floating set-ups are pretty much unrelated - you can have either, neither or both.