I guess it can be perceived as kind of annoying, but I can also see why people do it. If anyone says anything on an internet forum (especially when dealing with favorites or top ten lists or predictions), most people who disagree will go for the jugular and try to dissuade or antagonize the person in very hostile ways. The whole opinion things seems to stem from people wanting to share their opinion (and from some I've read, even asking of other's opinions) without the unnecessary flaming. It's like an asterisk or some kind of warning so that people reading the post don't go in with a bloodlust.
In an ideal world, there wouldn't be a need to state the obvious (again and again), but this isn't la-la land. People don't want to be flamed every time they share their opinion.
