If the standard makes ALL leaders corrupt it doesn't favor democratic over dictatorial ones, nor is it a very useful standard.

Useful for what? I agree it's not terribly useful for choosing whether person A or person B should hold role X, but I feel that question is a distraction- your design of role X is more important than your selection of a person to fill that role. And so the question of how someone acquired power is less interesting to me than the power that person has, and I think the link between the two is a lot weaker than people expect.