I don't know what it is exactly. I just don't like the fact that I have to do more work to be given a discount when it could have been given during checkout. The whole system doesn't make sense from a consumer's point. From the company's point of view, it makes complete sense.
Plus there's always the risk that you filled something out incorrectly, attached the wrong document, forgot to attach something, it got lost in the mail, etc etc.
And for some reason, I'd trust Apple with a mail-in rebate more than any other retail company out there. It would make sense that Apple would have it back on time.

You can't say the same for all the other companies out there though...