Nick: I would submit that in the vast majority of all-in situations, there are yet cards to come, so the decision as to which player has the best hand at the end is yet to come.
For example, last Wednesday, I was playing cash and two players were all-in pre-flop, one had AA the other KK. Obviously the AA is ahead by far. As I recall, the flop was Q, T, A, giving the player holding AA a HUGE lead. However, the player holding KK now has the straight draw possibility. The Turn was a blank, and just as though it was on TV, the River was a J. Over $400 pot with Aces again cracked.
What does this have to do with whether the cards of an all-in player should be turned up, NOTHING, at least not that I can deduce. However, as I said before the reason for putting the cards on their backs in an all-in situation, at least in my opinion, is to prevent chip dumping and/or collusion between players.
I know of several people that used to frequent my local card room that used to play as a team. They would get someone between them and whipsaw them out of their chips and then at the end of the night they split the good fortune.