And that statement, while better than how it is presented the vast majority of the time, still
isn't sufficient to make the answer unambiguous. There is still the question of Monty's motivations which could make it either adviseable to switch (up to 2/3 odds of winning) or to stay (up to 100% chance of winning).
An interesting follow-up on Marilyn. Later she tackled a restricted version of the problem that I presented at Spooky math problem (the restriction being that the two envelopes hold money, one has twice the money of the other) and correctly analyzed an argument for whether you should always switch. But she incorrectly analyzed whether you could do better than even odds. I know a couple of probability theorists who pointed out her mistake to her, but she never admitted to her mistake.
Make of that what you will.