I'm going to have to go ahead and agree with Joseph here. There's not a lot we can learn from sending people to Mars that we can't learn from observations from orbit or from the rovers. Mars is actually a pretty boring planet in the grand scheme of things, and there's plenty of more interesting rocks flying around in space. I actually went to a talk a few weeks ago about some of the larger rocks in the Kuiper belt (Pluto and a bunch of the other minor planets). Turns out we've actually learned a lot about how orbits (and subsequently solar systems) form. In summary: the time, money, and manpower we're putting into trying to put someone on Mars is way out of proportion with what we'll get out of it, and those resources would be better spent developing new instruments for observing other things in the universe. (Also, the idea terraforming Mars doesn't really make any sense, so this whole "backup Earth" thing isn't gonna happen either)
Oh, and bonus points if you remember where my post's title comes from.
