Not sure it's even good in theory - frankly, if you dislike something so much that you have to express it, you can leave a comment.
Facebook is, functionally, a social network. The infrastructure's there to promote social interaction - which is in itself the reason they've made it very easy to casually celebrate one another's outbursts, and a little harder to casually punish each other. There's already enough meanness, negativity, anti-social behaviour on Facebook. Absolutely, you should be able to express a withering opinion on what your mates have written, but for the system to actively encourage nonchalant meanness would be beyond stupid. Because a "dislike" button, at its core, would just be an easy way to make someone feel bad without having to confront them. Completely against the ethos of the websites. For all it's flaws, Facebook's there to promote discussion and interaction, not to put it in front of a jury.
The way it is now, anyone who has an issue has to put their head above the parapet, and expose a little bit of themselves as well. They don't get to sit around in a comfortable tower, sniping away. That's a good thing. You're still able to disagree, you just have to mean it - and maybe, this way, with your profile picture staring back out at you, it'll make you think about the way you express it.