Quoting someone's .sig:
A: Because it messes up the order in which people read messages. Q: Why is top-posting bad? A: Top-posting. Q: What is the greatest impediment to readable messages?
Save it for games of Jeopardy, please.
And as for the argument of having to scroll through screenfuls of quoted text, that's the result of people being too damned lazy to trim quoted text appropriately before adding (or interleaving) their replies.
If you're not willing to take the time to make your message readable, I'm not going to be willing to take the time to decipher it.
See my .sig.