I will never limp AA or KK. My goal is to raise and trying to get to a 5-bet all-in. By the time that we are 5-betting, there should be only a few players active, which is what I'm going for too, just to avoid the board running in their favor.
But if I am in a tournament or should you be playing against smart, thinkable players, I would sometimes limp. For example, say I have a loose player as the only opponent in the pot with me. I might call if I know he will raise to push me away. Then I could either call or raise, but because I know he will c-bet but probably fold to a preflop raiser, I will only call. Deppending on the Flop I re-evaluate. If it's a wet board, I will raise his cbet or bet myself. If it's a dry board with high cards, say KJ5 rainbow, I will raise his cbet hoping he caught a pair. Then follow some strategy for the rest of the hand.
Point being I'm limping with a Premium pair hand against players that are capable of giving me money on the Flop whereas they would fold to my raise.
Another reason is if I am playing against smart players for a long time. If I never call with AA or KK they will take those hands off my range, and insert them when I do raise, and since they would be spot on, I don't want that. So, although you should raise, say 3 out of 4 times with TT+, I would make a call here and then. If we do get drawn out because of that one call, we got unlucky (it would be easier if we got drawn out the other 3 times we raised).
But in order for me to call I would want a pot with 3 opponents max (and more than 2 are risky). If there are more than 3 limpers, I'm raising to push some away, no matter what I said above.