The free users of Calibre Cloud are using the freely available app. If CC was free, they'd use that instead.
I think, given the choice of connecting via wireless/content server in CC, and importing via save to disk folders, most people will opt to store lots of books via wireless/content server. The ones importing from a saved folder will be doing something special.
Regarding polish. I am sure you will be able to cook up something simple yet nice.
There is no need to build a full-blown cloud interface to the calibre db just to give this feature polish.