I like my personal Findory blogs feed. Serendipitously, I run across some interesting posts from feeds I'm not subscribed to. However, I don't need Findory to tell me about new posts to plasticbag for example. I'm already subscribed with Bloglines.
Of course there doesn't seem to be a good reason why my aggregator couldn't tell Findory that, other than that there isn't any way to do so.
Take this request with a grain of salt though. First, I'm probably an outlier power user who uses Findory and an aggregator, whereas Findory is aiming to be the primary aggregator for many folks. Second, providing a well designed, well performing API is not without challenges. Although as Linden points out, for some tasks so what if they're hard, the're worth doing anyway.