Some niche titles with small publishers and a small loyal following can live with no copy protection at all without being pirated to death. See
Thirdwire games for example. You pay with paypal or other, download the game, install and play. No copy protection at all, not even a serial number.
But for small publisher that attract a lot of following, copy protection is needed even if the effect might be marginal (because games get pirated anyway).
World of Goo is a good example of that, small publisher, huge number of fans, no copy protection, you'd think people would support that with their wallet but the game was pirated to death.
For big publisher with blockbuster games choosing some form of DRM is obvious even if the net effect is hard to quantify. But, assuming that games will be pirated no matter the kind of DRM, they need to find a good balance between "hassle for the pirate" and "hassle for the legit customer".
I don't have figures, but I don't think that games with simple protection like a serial number are more pirated than games that require online activation.
GTA IV required the user to register accounts on 3 different platforms, and it was pirated right away. The system is more problematic for the legit user than for the pirate who just needs to install a crack and start playing.
Spore is the most known example, it had a very restrictive DRM and yet was the most pirated game of 2008.
http://en.wikipedia.org/wiki/Spore_%...29#Controversy
In my opinion a solo game that requires permanent connection and account creation is unbalanced and creates more trouble for the legit user than for the pirate. I suppose we'll have to wait the end of the year and see how Assassin's Creed 2 for PC sells vs how much it's pirated.
Also worth considering, many games have their DRM removed with the last patch, basically when the significant sales are over. So if the DRM is so unbalanced as to be a hassle for the customer, people will delay their purchase and buy a DRM-free game a 1/4th price, not good for a publisher either.