I have some of the time been limping in limit heads up games those times I have a person who raises every limp and maybe 3bs every open raise. My reasoning is that it gets two bets (instead of three) in with the weaker range vs his whole range and three bets (instead of three to four, the four possibly not being in ones strategy anyway) in with a limp reraise with my stronger range vs his whole range. I have just as often been just taking the 3b up to every time, open raising the whole range, just folding preflop some more as it makes my range significantly stronger postflop.

I limp for similar reasons in big bet poker that I have been building in micros the last two years but not so much with a reason to limp rr at this time but i don't like to play a bigger pot with a weak hand when i know i will get 3b with a dominating range.

Some players might raise every time when one limps, making them good targets for trap limping. In ring games they don't need to raise every time for it often still being better to limp.