They're fucking stupid and discourteous to content providers, largely by virtue of their incessant usage. Using them to flavor a text is one thing, but responding day after day, post after post, with nothing other than an emoticon demonstrates that you are either unwilling or unable to actually make a cogent comment. It doesn't even matter if it is particularly insightful or complimentary or what.
Posting nothing but smilies all day long makes you look like a fucking simpleton.
Straight to the point!