Given that you'd be rolling with a net +5 penalty if you went with the punch chart, it's something you'd only want to do if you faced a target that was shut down or unconscious. Any other time, you're looking at some outrageous hit modifiers. Let's see, a Mjolnir piloted by an average pilot runs and attacks a target with a +3 movement modifier- that's 5+2(running)+3(TMM)+1(Mace)=11+ to actually connect. Ouch!
Obviously, most people will be putting a really good pilot on a mech like a Mjolnir, but it's still not something that you're going to want to go swinging wildly around, especially with the penalty for missing.