> just because they are not as good at you at seo?
It has nothing to do with SEO, and everything to do with convenience. Think about early /.. Since I enjoyed reading the same kinds of things as cmdrtaco, not having to Lycos (hehe...) news was a pretty big value add. I can go to one URL, get all of the tech news, and save a ton of time.
As far as determining if something 'has value' or not, we can just look at the facts. If an aggregator wasn't adding any kind of value, nobody would be going there. It's not like buying a URL and putting up some ads means you make instant cash. You still have to get visitors to come to your site somehow, and you do that by providing value to your audience.