I think that if more publishers put out demos of their products on PC as a mandatory thing, that would lower the percentage of people who pirate their games. For instance, I just upgraded my PC - not to anything spectacular, mind you, but enough that I am curious to see how the "juggernauts" of the platform perform.
Crysis has a demo, and that's great. That being said, GTA4 has earned a pretty notorious reputation for being a resources hog depending on what it's being run on, and I'd like to see how it functions for me. If it runs well, I might consider buying it. There is not, however, a demo for GTA4. So how am I supposed to know how the game will run, short of taking a risk and buying it? I don't want to buy a game that runs like shit, or doesn't run at all.
There's probably a significant percentage of people who pirate games strictly out of curiosity. People who don't want to spend money, but want to see what the game's all about, and aren't provided the luxury of a official trial version. So, the "trial version" they end up acquiring just so happens to have the entire rest of the game with it.
Every big-budget game on the PC needs a demo. Period. A lot of developers don't seem to like making demos. There seems to be a purveying attitude that certain huge franchises don't need demos because they're "known quantities", and they want to force others to buy them sight unseen (because, well, of course it's a good game, take my word for it, you should get it right now).
That line of thinking needs to go away. If you are publishing a top-tier franchise on the PC, and especially if you are pushing graphics technology reasonably well, release a demo. No argument. You may not see a dramatic boost in sales but you will see a reduction in piracy.