usermod as non-root user

I often want to run important system-administration operations without the capabilities needed to affect anything in order to verify that my command line is correct before getting the capabilities.

Say you're trying to add yourself to a group for managing a new piece of software. How do you make sure you don't remove yourself from "wheel" accidentally, leaving yourself unable to apply critical security patches in a timely fashion? In order to be safe, you should be able to run the command in a "pretend" mode that will report the actions it would take without actually taking them (it doesn't seem to have it, so the rest of this is kind of theoretical). But if you're worried about making a mistake in running the command, you should be worried about making a mistake invoking "pretend" mode; to be safe, you should run it without the capabilities to cause actual changes, verify that the effects are what you want, and then acquire the capabilities to do it for real and run the same command.

As a policy, prohibiting attempting operations without appropriate permissions just increases the chance of security breaches due to administrator error.