Sure would be nice but i would rather have watch stations on the moon where the glow from Earth would not affect it. Being able to do something about it though would be nice with out Nasa... Destroying one is worse because then it becomes a shotgun blast. Deflecting it is what it is all about and gee whiz, NASA is working on that right now.
The asteroid tracking programs (LINEAR, NEAT, etc.) are all small and under-funded. Again, if you're seriously worried about the thread of a collision, you should think about tackling that problem before you start planning to evacuate 6 billion people to Mars.
As for space-based observation, I think orbit makes a heck of a lot more sense than a telescope on the moon. Do you know how insanely complicated it would be to land a large lens on the moon intact?