They definately can and they should - 1ms of GPU time is really cheap, its only 0.5fps loss in 30hz game, its really nothing. I would probably accept game that runs in constant 29fps and has than 30fps and aliased.
It has to be said, looking at the results from FXAAv2 and FXAAv3, and assuming the performance figures we've seen are representative of those you'd get in a real game, it's beginning to look like a no-brainer to use it in any new development projects. I'd be very disappointed to see any of the AAA titles due out at the end of year with weak or no AA.