Here's another reason why limping can be bad for you. Say you limp everything except for monster hands like A-K, AA, KK-JJ. Then you raise pre-flop. Most players, even the dumbest of idiots at 2nl will eventually catch on. So when you have a good hand, you won't get paid off. If you limp with AA, you're setting yourself up to get beat on the flop by a crap hand, say 7-2o when the flop is 2-2-Q.