Well, the AT&T femtocell only allows registered cellphones on it, so the issue about bad connection or eavesdropping is irrelevant.
It only allows AT&T registered numbers on it. It doesn't care who those numbers belong to, as long as they're with AT&T.
I have two of these devices at work and after getting them registered (using a AT&T number on our corporate account) I've been able to add any AT&T number I've tried to them, ranging from other people on the corporate account to clients or guests who were just visiting the office for one day. As long as it's an AT&T number, it will work, and it will make calls using the device.
So I'd say eavesdropping would definitely be a concern, if calls could be handed off from a normal cell tower to a micro-cell.