Everyone bringing up World of Goo and the first Humble Indie Bundle need to remember a couple of things: firstly, that the statistics you're quoting are merely percentages of the total, and say nothing about actual sales figures; and second, that a pirated copy does not directly equate to a lost sale. People pirate games for all sorts of reasons, but one of the big ones is just that they either do not have the money in the first place, or they are not willing to spend it on the game in question. These are not people that would have bought the game had piracy been impossible, and this is especially true of indie hits like World of Goo, which deviated sufficiently from mainstream gaming clichés to put a lot of people off if they had found they had to risk their cash on it before knowing if they were going to enjoy playing it or not.
The same goes for music, film and every other media that's suffering piracy troubles at the moment - I know plenty of people with music collections that would cost hundreds of thousands of pounds to build legitimately, but the music in question is not stolen for any sort of commercial profit; it's stolen for the love of the music itself, based on the simple fact that enjoying it legitimately is prohibitively expensive and the missed sales would never happen anyway. It's a balance every company has to try to strike on its own terms, but at the end of the day, commercial entities are not interested in piracy figures as a percentage of sales, they're just interested in sales. World of Goo sold well, but if it had been riddled with DRM it would in all likelihood have sold fewer copies than it did, because the pirates would have still cracked it, the people who wanted to steal it would have done so anyway, but the legitimate buyers would have been less enthused about it. Maybe piracy would have been lower, maybe it wouldn't, but based on the evidence of the last decade there's precious little reason for companies to keep making things tougher on the legit consumer. How many potentially great games have suffered commercially because they've been blasted by the gaming community for their obnoxious DRM? How many more sales would Assassin's Creed II have made if it hadn't had all the bad press it got pre-release? We can't know for sure, but we all know people who have refused to buy games based purely on their DRM. Those are the lost sales that the big developers should be chasing.
I'll never advocate piracy, for the simple reason that people deserve to be paid for their work - but at the same time, I reserve the right to choose only to play games from developers that treat their paying customers with a modicum of respect.