I raise all PPs from all position in six-max, and call if I'm getting at least 15:1 in implied stack odds. The reasons for this are:

1. If everyone folds you pick up the pot right there.
2. If you miss you can probably cbet your way out of it.
3. Flopping a set in a limped pot is a waste of time. Nobody gives a crap about the 3-4 BB in the middle. In a raised pot people will move money to the middle with air, they'll be much more likely to have strong top pair, or 2pair hands, flush draws etc. You're looking to play for stacks, and the best way to get that to happen is to put something worth your opponent's risk in the middle. Imagine this. You're in the big blind with 22, and everyone folds to the small blind who completes. The pot is 2 big blinds. You flop a set, you pot it. Your oppoenent calls, and on the turn the pot is 6 BB, you pot it again, get called. On the river the pot is 18 BB, you pot it again, he calls, and you win a 54 BB pot, for a profit of 27 BB. Now imagine if you'd raised 4xBB, got called, and pot'd every street. You'd have paid about the same preflop to win about the same in terms of odds - ~24:1, but the win margin is much larger, and the scenario is much more likely to happen.