When playing poker, you should never let limpers get into a pot for a small fee. When you got good starting hands you should always place a big enough bet to scare players away that otherwise would have called if they bet was too small.

Here's an example why you shouldn't :

Lets say you're the small blind and you got a pair of aces in your hand. A couple of guys behind you call, it's your turn and you decide to place a bet, probable about 2 to 3 times the big blind (lets say blind levels are 50/100 and you bet 300).

Most likely some of the players would fold, but there's always players who will take a change and are willing to pay a small fee just to see the flop.

For example one of the limpers got 6s 8s, he/she calls your raise and when the flop comes he hits a 2 pair (or worse a straigt / flush). You will lose your hand because you let him limp into the pot. The best thing to do with your pocket aces is to make the bet too steep for limpers to go with, and if someone decides to call, it will be more likely he has a big hand also but will probably lose to your aces.

You should also remember that your pocket aces (or Kings) will almost certainly win against one opponent, but the more opponents in the pot the more likely that someone else will get more then just one pair and takes down the pot.