Addy Santo suggests that aggregators support full text searching:
Is anyone out there willing to take on a real challenge? Then how about adding full text indexing into your aggregators!
Yet another advantage for NewsGator…full-text searching through all of your saved news posts. Or even do complex searches involving certain fields, categories, date ranges, etc. And if you don’t want to limit your search to news posts, you can search your email at the same time, too. :-)