The large database cost is noticing that people who follow JGCs feed also tend to follow the feeds from the Chris's, furthermore they're apparently real human beings and have been reading for awhile, therefore when optimizing search engine results I'd put something linked to by JGC much higher than something linked by rgfbebgep240895i who is probably a SEO spammer rather than a human.
If you'd prefer a physics example, admittedly popularity is not truth, but you Might be able to add valuable data to search engine results by a careful examination of exactly who subscribes to Shetl-Optimized vs Backreaction vs Not Even Wrong vs all of them, and how long people subscribe, and other blogs they read...
Its the relationship between my set of feeds and your set of feeds that "should be" useful, not just the feeds themselves.
Something no one has discussed yet is the whole topic might just be a big F you to the NSA. I'm sure they'd love a subscriber list of "from the bunker" or "zerohedge" and GOOG has to either give it or give up on the product so they have nothing to give. Can't give up on search results, too much money, but reader might have been expendable.