I don't "like" it, it creates popularity contests, people will be less inclined to write the unpopular criticism or say the things that needs to be said sometimes. There will be groups and friends of.. outvoting one another, people get upset for not getting thumbed up and people's ego's get in the way. It's the internet, it happens.
Anyway on most forums there is simply the "+1" comment habbit (simply quoting and typing +1 (or +1000 if you Really agree..) or QFT, Quoted For Truth, habbit) if one really wants to reinforce a comment by someone else without having to write anything yourself.
But I do not understand that, if something you agree with is already said then ehh..then that's great. Guess it's really a sign of the extravert "social" personality culture that people feel they need to make themselves count and heard in any situation whether they have anything to add or not. (And this comment would probably get thumbed down it's face right off, mhm.)