I think the app ratings for the popular apps are all useless (they're well know apps, other people rave about them, so no need to read all the reviews). I agree on the long tail being useless as well.

Instead, I think it's most useful for the "middle tail": those apps that do get a fair bit of usage, but are not considered popular: the developer may not have build up a reputation yet, but the reviews help confirming the application does what it is supposed to do (or not of course).