yup, it's one thing to have a very small object essentially falling down every time it takes a step but for a large multi ton machine to do it, it just puts a lot of unnecessary strain on the frame and mechanisms... which has a lot to do with why our large scale insect like robots tend to move very slowly, at least the ones that try to be stable. The ones that have abandoned the idea of stability can replicate the quickness though... but again, not ideal for a weapons platform.
Well, unless you could have a computer basically predict how the frame was going to fall and then auto correct the aim on the weapon even before the frame moved thereby creating a stable mount... err... or you could just have a more stable mount to begin with.