If humans would care more about animals BEFORE they all die out, then there wouldn't be a need to consider reviving them!
Whether humans cause it or not, once an animal is gone from this Earth, it needs to stay gone. Would seeing a living mammoth be cool? Yes. Would it help keep people from starving around the world? No. There's no telling how much money would be spent to simply research an animal that shouldn't exist anymore. That money could be used for much better things.
Also, though they would claim it was only for research purposes, scientists would want to keep pushing it. If they bring back 1, they'll bring back more. Then they'll want to see them breed, then they'll want to see them interact with other animals...before long, a creature that died out hundreds of years ago has been released back into the wild to resume where it left off. Who knows how that would affect creatures that are still alive and well.
I think the question is this: Do human beings REALLY need to challenge the process of life and death? If it's gone, it's gone and it's too late to do anything about it.