I suppose it kind of begs the question, if they're thinking this all the time, and when they let their emotions go, the truth actually comes out? It's like the saying that, there's a bit of truth to every joke, or the idea that you find out who someone truly is, when they get drunk.
I'm sure it's to make you feel hurt, and make you feel like your in the pits, just because you're in the argument. And sure it's easy to say that that they don't mean it, but like anon said, if you didn't mean it, you wouldn't say it. Having said that, I've said things that I really don't mean, but it doesn't mean that I haven't thought about it, even in the slightest.
I guess the argument situation, is a place where the true feelings come out to roost, because at that point, you really don't care how your words will affect the receiver (it's only after you realize the damage that you have done, and then start feeling guilty and start back peddling).
Is he / she saying these things to be hurtful; no doubt about it. Does he / she really mean it; quite possibly...