Clutter posting -- or, you know, spam -- is one of the main reasons I like the rep system. That little green arrow, to me, is a way to say "noted and appreciated" instead of taking up space on the page with statements irrelevant to the discussion. Not to say you can't express appreciation and then continue the conversation, but many times there's a post I liked but I didn't want to post for a variety of reasons. Upvotes are a good solution for that, I think.