So, some reasons for adding (not sure why I really have to provide this, but I will):
- There's times where I find I want to show my appreciation to someone for their comment in a topic and that I agree with them; however, I don't really have anything to add to the discussion myself, so the "Like" button would be a great way to show this appreciation.
- Application example: in the Help and Advice forum, where a "Like" could better show a refutable, correct source of information.
- Specific example: http://forum.tip.it/...st__p__5232557. I want to show Miss Lioness that I agree with her and I also didn't know about the event, but as you can assume that wouldn't add to the discussion much, especially since angel13 below her has already told her that they agree as well (so, to comment saying I also agree would get rather annoying with so many people agreeing).
In case someone was pondering any downsides to this, there aren't any: since there isn't an "Unlike" or downvote button there isn't any concern for people gaming the forums and hide a post, everything would purely be positive.
tl;dr: enabling the "Like" button would be great for showing that you agree with someone, they're providing accurate information, and appreciation.