I'm not sure what you mean about using the front-end without the back-end, but Inky's an IMAP/POP client like any other; it just stores your settings in the cloud. (If you mean "can I use it without storing my settings in the cloud", the answer is currently no... at least, not yet.)

We are not parsing the mail and mining data from it, nor do we plan to. We're looking at various non-privacy-invading ways to make money but we need more real-world data to see what makes the most sense.

I don't necessarily know how well you are protecting them, or if your admins are able to copy my password. Even if you encrypt it, who's to say how well? What if you DB leaks, and my credentials are in the hands of the "bad guys".

It's not fundamentally terrible, but it increases the attack vector..

We're going to add this as a FAQ on the site with the next rev.