All excellent points. Also, we have to factor space combat in something like Gundam, which is where the humanoid design could potentially thrive, and even scale up, though it might be unnecesary. Smaller mobile infantry would be effectively challenging to hit, but larger Gundam-esque craft would be able to load as much armor as their inertia-to-thrust ratio would allow. The ability to grapple and dexterously propel would serve a purpose against larger craft, assuming it could catch them.
There is still an issue of fuel and propulsion, however, because as a species we haven't publicly mastered any viable technology for something more extreme than clunky NASA travel methods.