I think the technologies developed to make a mars habitat possible we end up also being useful for making some parts of a future earth habitable. That's a coincidence I hope doesn't come true.

Personally, I think every billion destined for mars exploration is money not being spend on addressing the more pressing crisis: What about the health of our current planet. Mars can wait, so to speak, for us to ensure our survival here on earth. A mars colony probably wouldn't survive a collapse of earth-based civilization.