They use them for proper creative work, not looking at Facebook and lolcats.
I've been waiting for a decent Mac Pro revision (yes, the tower machine, not the Fisher Price VHS/DVD combo imacs) for 4 years. If it doesn't appear soon I'm going to switch to a PC or Hackintosh, as I desperately need it for work.
I've always been happy with the power of the iMacs, personally, so I guess it depends on what you do (although that's still no reason to superciliously dismiss iMacs as Fisher Price toys), but I read the reason the Mac Pro has gone so long it may as well have died completely is that there hasn't been an appropriate upgrade to the chip family it uses that would warrant a redesign?
But then like I say I don't even know what people use them for (that's not a criticism, I genuinely don't know how they are used and what they are needed for) so I don't know how valid a reason that is.
3D, rendering compositing, colour correction, motion graphics etc.
Imac is ok for web and graphic design, but not very upgradeable and not powerful enough.
Saying that, most post houses are all PC now. Apple have made it clear they don't give two shits for the pro market.