WARNING (and I apologize for) A WALL OF TEXT...
Hohoho, an interesting thread while I'm away, and look, it's already 3 whole pages of posts. This topic have been discussed over and over again in the past (not here), and it's really fun. Is bipedal humanoid (gigantic) robot/armor/mecha (or whatever you call it, you philosophical dude) could be useful in real war? The short answer is NO.
From the shape, humanoid form is a bad form for a battle. Without tools, we human will be overwhelmed by almost all animals out there. And from the sky, standing humanoid form is a nice target for missiles. From the ground, standing humanoid form (especially for giant robots like gundam) is very easy recognizable (and I bet humanoid mecha is a valuable opportunity target).
Bipedal also a bad idea. Past anime/game always mentioned that bipedal advantageous is to be able to travel all kind of terrain (especially compared to tanks). Actually, it's the other way around, tank threads is way better than bipedal. Using bipedal, meaning the entire weight of the mecha will be focused in two tiny spots (while threads spread it evenly). The only benefit that bipedal has over tanks thread is the ability to "walk" over tank traps. And also don't forget that with a right weapon, infantry could easily achieve mobility kill against bipedal mecha.
Having a hand is also another bad idea. Since complex operation needed to just fire a gun (aiming the gun, pressing the trigger, and then the whole mechanism of the gun itself). The weapon size, caliber, and recoil is also thing to consider.
And also don't forget other things that plague a real battlefield (especially if you are away from the homeland). Things like maintenance and how to deploy those mechas into battlefield (and how to extract them from battlefield).
In the future, most battle would be fought in city (Iraq, Afghanistan, etc) and city battle is the one take most causality. Tanks is very vulnerable in city battle since there are too many blind spot. Anyone who played Battlefield 2 should already know about this. Tanks without infantry support = death wish. I think the future of the battle lies in infantry. That's why US future warrior program is all about infantry, right?
Instead of giant mecha like gundam, it's better to have smaller bipedal armor suit, like those in Alien 2, Spaceship Troopers, or Avatar. Instead of treating it like a tanks, it's more like infantry, but with abilities to carry all kind of heavy weaponry. Who needs one tank if you have 4 infantry (one fire-team) that could carry heavy caliber machine guns, anti-air, anti-tank, anti-bunker missiles, and all other kind of weapons... and equipped with all kind of sensors that a tanks would normally have? It's also suitable for city battle (since you could still use building for covers).
Anyway, one thing that really important to solve right now is (just like other said) the power source.
Mecha, if human could build it, is more useful for building/construction purpose, I believe. Just like we, when still a child, building blocks and other things. Patlabor would be a good example of this.
And one more thing to say... and it's that makes me wonder, why all games and movies fail to address. If we already could make awesome war machine like gundam or other mechas, why we couldn't make even more awesome tanks or fighter jets (or even battle cruiser/aegis battleship)? All this time, all games/movies only pit a ultra high tech mecha against obsolete tanks or fighters from stone age...
And considering all things I mentioned above, in real war, it's even better if we just deploy multiple latest generation tanks and fighters (all remote controlled like drones) compared to a single giant mecha... it's cheaper and more effective.
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