Nick: Very interesting question, and one the TDA has been and will continue to ponder probably forever. Let's look at the details:
1: First and perhaps most important, if you want your house rules to require all hands at showdown to be tabled live, you certainly may... "except where house policy requires a hand to be tabled...". SO, if your policy requires it, just make sure your players are aware and you're fine.
2: From the player's standpoint I can tell you that the vast majority of players DO NOT want to be required to show their hands at showdown. Also, the vast majority consider proper reading of your own hand to be part of the game and if you mis-read your hand and fail to see you're the winner and instead fold, that's a defect in your skill-set.
3: At Summit VI Neil Johnson made a revealing comment, something to the effect of "...I'd like to see all cards at showdown turned up live and that would do away with all this showdown stuff... but can I get even one player to agree to it...". That really sums up the player position on this IMO.
4: Philosophically there certainly is precedent for all cards up at showdown: first in online poker where they always are, and per TDA rules in all-in situations. The justification for doing it in all-ins has been well-explored previously.
5: This leads to another related question which is whether just the winning hand must always be turned up, even if all competitors muck face down at showdown. Again, there are TDs and players on both sides of this issue so consensus will be difficult to achieve, if ever. So for the moment, if that's your house policy, the sentence you quote permits it. On the other hand, turning up the winning hand is not a requirement of TDA rules if there is no competing hand tabled.
.... And BTW, I'm personally fine with either no hand required to turn up, or just requiring the winning hand to turn up. I understand the philosophy of both schools and they both have good arguments, I lean a bit more towards not requiring it, but certainly can live with it if they do. Going further and requiring ALL hands turned up, I'm not as excited about that one but recognize the rights of your house to use that if you wish.
This debate will continue, no doubt....