When playing 6 or 9 handed, consistently limping with trashy hands does not tend to be a profitable play. You just don't hit often enough for it to be profitable.
I am more likely to limp with junk in late position if the pot odds
are too juicy, but I won't hesitate to throw away top pair on the flop if the texture is real scary.
If I'm playing heads-up or even three-handed, sometimes I'll limp- particularly if I know that the other players will let me see the flop for cheap.
I gotta admit though, it is fun to bust pocket aces with something like 2 8 off suit.