The excuse of most slsk users surely, "I was trying before I buy"
Well I was playing devil's advocate really but the point is that the cat's out the bag wrt p2p so it's kind of silly to pretend that it doesn't exist.
Slsk isn't the best example anyway because it has traditionally been the roaming ground of the voracious music lover rather than the casual downloader. I would say in general people on slsk buy a lot more music then those on most other networks. I hardly ever use p2p now but when I did it would always be either:
a) In lieu of being able to get what I was after - pending a search for some hard to find item.
or
b) To check out some new/old hype - which I would attempt to buy if I really liked and happily delete if it turned out to be of no use.
Both of these mitigated by the fact that I buy music at (slightly above) the rate I can afford to. Of course this has been endlessly debated so I'm just talking about my scientific sample of 1 - myself.

I've never bought an mp3 although I have considered getting that Kode 9 & Spaceape Fukkaz one because it isn't available on any other format.