I think the colonization of planets is particularly beneficial to the human race and life as we know it because it gives us entire planets to test how life works. In that, I mean, if we were to successfully terraform a planet into a habitable environment similar to the once pristine and natural conditions of Earth, we could learn out of the world (hehe) concepts about what we don't know already, as far as things like evolution and natural/unnatural planetary development go that we're not already aware of. Concepts are one thing, but being able to research a developing biosphere over the course of generations is, in my mind, as concrete as scientific testing gets.
Personally I see it possible. Expensive, in terms of resources used, and it would demand a collective focus of a major proportion of Earth, but hot damn would it be fantastic.
Plus, a back-up habitat is always handy. Imminent destruction of Earth? Send the most well-bred humans (as far as strength, intellect, attractiveness, etc. go) to a non-colonized (but habitable) planet! It's like a fresh, cheaty start!