Hmmm, I'm surprised you find that feature so crucial.
I find it almost negligible, certainly non-essential to the core functionality of the app, because, as mentioned, there are lots of other ways of getting books into Marvin, including from Dropbox. Most of us are celebrating Marvin because it is, at long last, a worthy successor to Stanza, already surpassing Stanza in many ways, but Stanza never supported Dropbox anyway.
That said, I agree with you that if a feature is offered it would be good to iron out any bugs in using it.