Thanks for the comments.
For some background, we are co-opting the Rover code to drive a boat having two thrusters and no rudder. So the differential drive of skid-steering does the trick. [To my knowledge, there is no accommodation for this setup in the Arduboat codebase.] Single joy-stick control is a more natural way to drive the boat, so that’s what we set out to do. So to be fair, we are not your typical users.
I think what we found confusing is that two-paddle skid steering on input is an explicit setting, while skid-steering on output is implicitly set by the servo mappings to the right and left throttles. Because it was not also explicit, It was not clear (to us) without looking at the code how mixing of the left and right thrusters (motors) would occur for steering on output.
If I might off a suggestion, I think what would make this clearer is if there were two new SERVO settings that would indicate explicitly “throttleLeft+skidsteer” and “throttleRight+skidsteer”. Then
have_skid_steering() could check for this instead. It would be functionally identical, but would better honor the “principle of least surprise”.