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Bloglines has a pretty mild level of lock-in, it's true. Thanks to export OPML, it's fairly easy to take your data to another aggregator. Even this hassle, however, is not negligible. If Bloglines has cornered the market for people seeking a simple, easy-to-access, no-installation-required aggregator, many of their customers might indeed not care to figure out what OPML means. Additionally, they have the usual lock-in that results from users being accustomed to a certain interface.

Also, as Mary points out, the historical archive of blog posts that Bloglines has is pretty valuable. I would say especially so for a search engine.