Note that they don't have to make a different user (as I understand it). They can simply log out.
I don't agree that it doesn't matter whether your opponent knows your strategy in a game of perfect information.
In fact, what your strategy is is exactly the information that they don't have and need to deduce.
Further, each player is imperfect (very much, at my level!) so information about what you didn't think of is extremely valuable.
But really there are two things more important that I don't want:
1) The nagging feeling that the opponent might be reading what I'm writing
2) The temptation, remote as it is, to go and look myself.
Why do we need Malkovich to be available to spectators during ranked games? Because that's what would be given up to solve this (in the way I am suggesting, at least).