I know that's the status quo, but I don't think it's fair or sustainable.
The only person who is gaining value from this in my opinion is the aggregator. Someone like digg which links to the original article is fine imho; but if they link to another non-primary source without original content/discussion then it's pretty murky waters.
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.
And I am certainly not claiming that doing any of this sans attribution should be permissible. Proper attribution is crucial and hyperlinks make it so easy that there is no excuse for e-plagiarism.