I also think we don't really need it, but with facebook trend, people want to "like" everything and tell they agree with anyone.
We started to have "+1" and "+1111111111" posts everywhere.
since we added the like button option, these posts are now history.
The like button/count could have been used to show how reputable and knowledgeable someone is, but people tend to like idiocy more than interesting information.
We can't base a reputation system on something like that.
I usually use it as a "thank you" or a "good to see it worked".
For example someone ask for help, I provide a guide, and the user reply that everything worked as intended. I don't always need to reply to a "thank you it worked", but I like to show that user that I read his thank message.
edit:
An example just came 2 seconds ago.
Maybe a "thank you" button would be better than a "like" button for a reputation/knowledge count, but users will unfortunately still abuse it.
Edit : maybe another idea to rate the knowledge and usefulness of a user on the forum:
instead of liking posts, we could
anonymously rate (pos/neut/neg) a user himself. only one note could be given per user.
but again, troll users will like other troll users, and that's not representative to the knowledge. it would allow bashing and negatively bullying a user for no reason.
but maybe with enough vote, the balance would be right.
but a system where we can set negative instead of positive note is always a bad system.