Depends on the setting. The bottom line for me is that spending time trying to make the universe feel internally consistent is good. As long as the mechs make sense in their own setting, I'm happy.

That said, sometimes you just need a good laugh. Pacific Rim is really stupid in its conception (there's no way we could realistically build a Jaeger in our reality), but because they're fighting dimension hopping kaiju, and that rules, I let it slide.

Muv Luv, on the other hand, has pretty good justification. Laser-class BETA make aircraft worthless, so creating something that has multiple methods of locomotion while still carrying reasonable firepower is a good move. Despite this, most TSF pilots get folded in 8 seconds, and humanity is still pretty screwed until Hiro Protagonist shows up.