When you have 100+ feeds you really want to avoid reading twice the same entry. It's the single most important feature in an RSS reader for me.

Agreed. The syncing can be managed other ways. The only thing I’m left with is using on a work computer for some reason, where one’s own devices aren’t available/permitted? But that’s probably not a common usage case.

So the OPML file does handle the read status as well? isn't it just a format to export and import feeds inside a reader?