Maybe a more experienced poster will be able to correct me, but I think it's mostly a matter of technique. Hold'em is a game of information and inducing mistakes, so if you think that you can confuse your opponent and have them make more mistakes by limping in, then it may be a good play.

However, against most villains you can probably induce more mistakes by either raising or folding all the time. You may get them to fold too often, or on the other hand call down too often. It depends on the player you are facing.

My biggest complaint about limping in (except when opponents do it), is that you give your opponent the choice to take a free card. In a game where even a weak pair is a strong holding, letting villain see the flop for free with foldable holdings can be costly.

You should read the section in HPFAP about open-limping on the button; it should give you some insight into what to consider when making decisions like this. While I sometimes limp on the button 3-handed (usually when folded to me, rather than a 3-handed game), I pretty much never open-complete HU. I like having an aggressive image, which allows me to get as many chips as I can in the middle when I have a strong hand.