I can't speak for anyone else, but I would trust a service like this a lot more if i could easily export all my data. I would do so on a regular basis, basically as a backup.

The other concern is what happens if you shut down or decide to change your plan to be prohibitively expensive? (I'm not implying that's the case for you, but it's a consideration that has to be made). There have been vendors in the past that have abruptly shut down without warning, leaving their customers scrambling to build or find alternatives. In all honesty, I'd rather build an authentication system from the beginning than having to take my service down at a critical time.

This is actually a really interesting product though, and i would definitely consider it on future projects.