Amiga would have been useful to Apple back in 94 when Commodore died. They could have used lots of stuff from the Amiga, form chipset to Workbench and AmigaDOS, and maybe OSX would be different.
Today, Amiga is useless to Apple. No new users, as almost everyone have the Amiga as hobby today, no technology, as they are already the top dog, and no new hardware... ...as we don't have it either :-?
If Apple have bought Commodore by the 80's, it is pretty sure they would had it running in parallel as they had the Mac and Apple II. It is quite possible that the Amiga would have killed the Mac, just like the Mac killed the Apple II.
We could have OS X running in Amigas by now...
(hey, I really like these Marvel Style "What if..." threads :-) )