I see it as a significant difference between "leg destroyed" and "leg blown off". In the former case, the joints are wrecked, nothing moves (or it moves freely whether you want it to or not), but the remains of the leg are still there, and you can put a ton or two of weight on it to help balance, even if it won't fully support you. At that point, it's about the equivalent to the classic "mummy" walk: step, drag, step, drag....
In the "blown off" case, the 'Mech is now reduced to "pogo stick" movement, and should probably take a piloting skill roll each turn that it moves at all, although possibly with only a +1 modifier, not with the full +4 "leg destroyed" modifier. In essence, you're making a series of tiny hops, not a giant one-hex leap on one leg, but there's not much margin for error in balancing.