Read Subscriptions are attention, but what about blogrolls? by Leslie Michael Orchard (decafbad.com)

Ah hah. Here's a use case where I agree OPML has undeniably become king: Exporting and importing feed aggregator subscriptions. Because Radio UserLand was the first aggregator to really take off—and because OPML is Radio's lingua franca, any new aggregators have needed to speak OPML to facilitate migration. It grew from there, with nearly every aggregator supporting some basic form of OPML import/export for subscription lists. OPML has won the "feed subscription list format war" before there was ever a notion that there might be such a war.