THESIS: There is no reason to not implement mouse steering for all types of wheels or even track (if you are into that sort of thing) as adding even one omni-directional part like Onmi-wheels is enough to get all other default movement parts to switch to mouse steering controls.
SUGGESTION: Add a new option in settings or in a specific blueprint’s control settings (like with weapon/perk key-binds) that allows you to set your controls to: Default/A&D Key Steering/or Mouse Steering.
EXTRA: I like to play with a Gremlin/Hermes build and find myself favoring the Hermit wheels. However, I practically need to place at least 1-2 Omni’s so that I can use mouse steering, as I think it makes it much easier to stabilize my car during high-speed ramming attempts.