Re: May 2011:The Discovery of Pandora/Earth 2.0: Earth 2.0 Announced!

[Human No More]

Mars is full of problems even if there was a breathable atmosphere - there is much less protection from radiation as it apparently lacks a magnetosphere, as well as it only being 0.38g on the surface (compare to the moon's ~0.16g and the problems apparent there). They do not make it uninhabitable, but they do create significant problems in doing so.

If Mars was a pure 'twin' of Earth though, with similar surface conditions over more than just temperature and water, then yes, there would have been far more effort to reach it. Perhaps planets like Gliese 581g will provide this impetus instead though
Having another planet would go either one of two ways - it would either allow for more of the conditions on Earth, or it would help rectify them as the existing population would have had more space, and the resources of both planets would have allowed to maintain a quaklity of life for the two populations without compromising the biodiversity of either.