I don't know a single poker pro who says limping is totally wrong. However, there are many factors that often make other choices more profitable. In particular, when you flop medium strength hands, they are naturally more profitable against a few opponents postflop than against many opponents. For this reason alone, you should often not try to go into multiway pots, but instead thin out the field early on. In addition, aggressive play gives you initiative, which can benefit you after the flop if your opponent doesn't hit - regardless of whether you hit yourself.
However, it's not fundamentally wrong to limp in from time to time. The situation has to fit. Against weak opponents, for example, you can do it with strong starting hands because they don't recognize the trap. If the odds are right, you can also limp speculative hands from a late position (small suited connectors, small pocket pairs). If you flop your monster here, the odds are relatively high that with four, five or even six opponents, someone else will have a playable continuation and pay us off over one, two or even all streets. Position is very important here, though, because you don't catch these monsters very often and you want to avoid a medium hand behind you checking back or folding to a big bet.
As others have written, new and inexperienced players tend to rely on this longshot strategy, which makes it easy to predict. They pay too many raises after their limps, in the worst case even out of position, then hit the flop far too rarely and have to give up. And when they do hit, they suddenly have to become active, which is a signal for the good players to fold their good to medium hands. Ergo: The permanent limpers do not get their fat hands paid out.
