I loves me my Mecha. Mecha have personality where tanks don't. You can say: "the 'Mech strikes a defiant pose before unleashing its weapons at its enemies," where a tank can't. Seriously, how does a tank strike a defiant pose? I mean, which of these two looks defiant (
Tank,
Mecha)?
Are Mecha realistic, or feasable? Not as we understand the future of warfare. It would take some applied phlebotium like Minofsky Particles or the Solar Reactors to make them more useful than tanks.
It doesn't bother me that in order to have Mecha they really do have to be powered by Awesome. That's just another facet of how cool I think they are.
Basically, in order for Mecha to be real, you have to have a fundamental reason behind it. And, as GM, you can just say "Mecha are better than tanks because, dag nabbit, I want them."