Small warning: major spoilers ahead

At the end of the main quest I sided with Yes Man. I destroyed all the robots (even when I didn't have to) and went for glory myself. Yes man tells me I have to blow up hover dam but it fails to give a reasonable explanation. Why would I need to blow up Hoover Dam? I can let the NCR and Ceasar's Legion kill each other over the dam, while New Vegas stays safe with me in charge. I think this should be a better ending when choosing to go for an independent Vegas. What do you think, why do I need to destroy the dam?

You don't destroy the dam.
But you have to be a part of the fight for it.

Reasonable explanation, you won't have the forces to overpower both Ceasar's Legion and the NCR. The only solution for that aspect is to sabotuge the dam, that way no one will want it and the NCR/Legion will abandon the Mojave. I'm puzzled as to why Legion would abadon it, I mean it would still techincally be another tribe that he could absorb into the borg, I mean Legion.