I have downloaded things in the past, and bought them if they were worth getting.
Example:
UFO: Enemy unknown. I played the demo, and couldn't work out how to do anything so kept getting killed. So there was no way I was going to buy the game. A friend got it, and lent it to me. I played, enjoyed and completed it. I then bought a legit copy of UFO, and have bought all the sequals.
So out of the piracy of one game, the developers have had about 8 game purchases they would not otherwise have had.
Same with morrowind, I borrowed it, loved it, bought it. And expansions, sequals etc.
With music, if I like a tune, I'll download the album. If I like it enough, I'll buy it. My rate of music purchase is independant of the availability of music to pirate, but thanks to piracy I end up buying the stuff I like the most. I will buy maybe 12 albums a year. Without piracy I may end up enjoying 7 of them, but with piracy I know I'll enjoy all 12.