Would you go on a manned moon mission? And why?

A moon mission - that'd be fun. Yes, I'd go.... As a kid growing up, one of the views I've always wanted to see was the Earthrise over the lunar horizon.

Because the seats on the manned Mars mission were filled up.

I would certainly go if qualified and can be of any help to the mission.

Absolutely. As for why, I say "Why not?"