I think (I've never pirated, so that's the best I can do) that they generally pirate games where they are unsure of the quality or believe is not worth the money.
For example, House of the Dead Overkill was pirated something like 800,000 times and I think some of that was due to people being not happy paying £30-£40 (at launch) for a rail shooter, and a fairly short one at that.








