IIRC, construction from existing instances in general is an issue in the current version of the module. One particularly murky question if it were allowed is what should happen if you pass an IPAdapter instance (which already has associated network info) rather than an ordinary IPAddress.

Forcing people to go through an integer or a string in that case, or call one of the explicit conversion methods, forces them to be explicit about the fact that they're deliberate discarding any other information associated with the original object. (Note that I like the 2-tuple idea - I'm just pointing out that allowing an IPAddress object as the first element of that 2-tuple isn't quite as straightforward as it may first appear)