The question why we should return to the Moon is almost the same question like why should we go to space at all, or why should mankind progress at all. Why don't we go back into hollows and even stop making fire.
It is a matter of decision and curiosity. If NASA decides to go to the Moon, okay. If they don't decide to go to the Moon, okay. Even if they decide to not go into space manned anymore, also okay. Just like Wernher von Braun and many others, I also like the idea of going to the Moon and to Mars manned, and that as soon as ever possible. Eugene Kranz, Apollo 13 flight director, says: "To stop in space is to surrender." There are two kinds of technologies: those who exist, work and are available, and those who are yet fictional and do not exist, less than ever work. Not to got back to the Moon and later to Mars, because we do not have science fiction propulsion technology available, is the most unwise argumentation. By going to such places with current technology, we learn and gain new technologies. The Moon experience is not over. It was just interrupted by Nixon and the congress. The initial path of NASA was the Moon, followed by Mars until 2000. Instead, the Shuttle was quite a sad delay in manned space exploration.
Constellation is the best way NASA has chosen after the Skylab program in the 1970's.