Gotta disagree, John. Sure, if you look only at the fact that one of the two unopened doors has a prize and the other one has an ass, the odds are 50/50. But there's more to it than that.

The MC is not randomly selecting either of the unchosen doors to reveal. If your first pick was an ass, the MC is ALWAYS going to reveal the other ass, never the prize. Statistically speaking, over a number of games, you are better off switching. Two times out of three you will have intially chosen an ass, and thus two times out of three switching will improve your lot.