That's a fair point. However, consistency and dependability are two important traits. I would not criticize anyone for resigning any position. Folks have their reasons. But neither is that something that should just be ignored.
Dependable means you put others ahead of yourself...you relinquish your power when it's in the best interest of the cause. There is no defense there for criticizing someone for resigning. Resignations are often very honorable acts. Not always, but quite often, especially in politics.