The players I know who illegally download do it as a way of previewing the books before they buy, because it's the kind of thing that can be tough to flip through in the store and really get a sense of.
I don't know how common this was, but around here the advance leak of the 4th edition Player's Handbook stirred up a lot of interest and reminded people that the first books were coming out soon. I've got friends who were really against the idea of a new edition, but decided to go ahead and buy the core books after looking through that leaked PDF.
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