I think it was one of the FFXIV interviews, the DEVs talk about how the Like button is the only way they can offset how rapidly people post about things they dislike and how seldom people post when they do (for fear of being flamed.) The Like button effectively offsets it because you can see how many people actually agree with a single positive post (a positive post may have 50 likes while there are like 15 really negative posts) -- this lets you know that sure, there are issues worth address, but there are also people who are satisfied.
The dislike button won't stop people from posting about things they dislike, because that negativity and anger will drive them to pressure the issue no matter what, so the dislike button would just skew the numbers in the favor of the negative sides.
Sorry about referencing the interviews, I forgot they are for FFXIV (same forum structure, though, so same basic principle) so you might not have seen them.
http://ffxiv.gamerescape.com/2011/06...iew-june-2011/
hit ctrl+f and put in: Matsui: Hahaha~
That's copied from a few lines above the section I'm referring to, should take you there.