Mankind is too rare and precious to limit it to one planet. I'm not one of those "the world is gonna end soon" type of people, but there is always a chance that the world could actually be destroyed by some sort of violent, cosmic force, it is very rare, but the chance is still there. Right now, Earth is the only planet known to contain intelligent life and we should treat it as the only place in the Universe that contains life, even though research and some speculation suggests there may be millions of planets like our own. It should be a fairly big priority to extend our reach to other real estate opportunities in the Solar System. Terra-forming Mars should be a first priority to allow humans, our culture and other species of life to live on just in case that slim chance the Earth is destroyed or uninhabitable, if NASA isn't around then that can't happen and that's pretty sad.