See, there are some really big guys out there. Opting out and throwing their own lawyers at the issue is a good option for them. And then there's the rest. Comparatively small advertisers, "only" spending a few hundred or maybe thousand US$ a month. Opting out and going to court is NOT an option for them. Quite a lot of them are outside the US - which makes it even more difficult. Before the settlement they were grumbling, calculating a certain amount of click-fraud into the overall equation. Now, with the settlement, they see the chance to get at least a LITTLE compensation.
My estimate is that more than 90% of advertisers have not other option than hoping that the settlement will succeed!