I'll add that I've been known to do this with AK from time to time, but I don't mind getting out on the flop if I miss. I think it can be disastrous limping with KK and QQ because you won't fold out junk hands that might randomly hit 2 pair or some funky draw. (Though the 2 pair is far worse) Furthermore, you encourage A-x hands to come along for the ride and beat you on the flop...
END RANT