Hi Joe,
I totally understand. I’m slightly regretting getting involved in this thread now 
Putting RoundCube aside, I suppose it just may be useful to have a Remote API for Usermin in general, for allowing any kind of third party software to do things at the user-level rather than the system-level like the Virtualmin Remote API allows for.
Of course, it would be totally reasonable for you to say that’s not what Webmin/Virtualmin/Usermin is intended for, and leave third parties to come up with their own workarounds.