A limper with an Ace will not pay you off if the flop misses him. Low PP will not likely play unless they hit a set - and then YOU are paying them off.

So - limp and let many people to limp in and stay in. If you hit the set on KTx or ATx or even QTx board - most likely someone will pay you off, sometimes with their entire stack. The more people see the flop - the more people you have to get a piece of teh flop and pay you. With AQs+ and QQ+ you want to isolate, with JJ and below early on - limp for set value.