You couldn't send credentials nor file descriptors to other processes without UNIX sockets. I would argue that allowing for that kind of decoupling of permissions and file access is very much in the spirit of the UNIX philosophy. Quite frankly, there are some problems you simply wouldn't be able to solve without them (in runC, the underlying runtime that Docker wraps, we use them quite extensively).

You can send fd's through UDS and they do expose a lot of info on the process on the other side.

http://www.thomasstover.com/uds.html