The thing is, driving a car (even a manual shifter) is much different from driving an 18-wheeler, which is much different from riding a motorcycle. Even among similar weapons, balance, timing, and usage may differ depending on design. A weapon without a crossbar will tend to be used differently than one without, for instance; the wielder could get his fingers removed using a particular weapon the wrong way. Stabbing with a dagger is different from using a shortsword similarly; you can do a lot more mobility-wise with a dagger, but you can do more initial damage with a shortsword. Daggers are better at extremely close/grappling range combat because it doesn't take as much room to get in an effective stab.
I'm pretty sure RQ2 had the same rule, where different weapons under the same category could be used at half the skill of the preferred weapon. As a matter of fact, consulting my happy red leatherish-bound rules copy, it does (p. 26, "Effects of Training With Similar Weapons").