If it is a very succinct and insightful quote, it can add quite a lot to an argument. But just quoting people for the sake of it, or the classic "they're-more-important-than-you-therefore-you're-wrong", gets quite irritating.
True. Let me give you an example just for the sake of it:
We're discussing about something, he/she says his/her point that is against mine, then I ask a question that says "you are contradicting yourself!," after which I get a quote backing his/her opinion. That's the most irritating situation.
We're discussing something, I express my thoughts on something, he/she agrees with me, I continue about the point, maybe going a little deeper or taking a bit bigger subject, something very related, and he/she quotes someone who said what I just said. That's the less annoying situation (as it's agreeing, so no need to "defend" against the quote), but it still is quote annoying.
I believe the first would account "someone famous said it, therefore it's true" and the second one for "I'm gonna quote someone because I think it's cool to do so."