@Jochem: I think Brian's point wasn't about expensive cards like foil money rares, but more about powerful cards in sealed format, such as Pack Rats.

If we allow a proxy under such situation (a card is registered but missing), such collusion could happen:
- the registrant puts in a couple of “bomb” cards in the decklist,
- and the receiver claims that those bombs are missing when he receives them,
- a judge issues a proxies for the missing bombs,
- receiver of the deck went on to sweep the whole event X-0..
using the “basic lands with the name of Bombs penned onto it”

This is what we want to prevent from happening.
:)

But in that case, why would the registrant not simply slip a spare Pack Rat into the pool? Why draw the attention of judges? (Not that I'm implying that cheaters are necessarily smart…)

Another possible scenario is that the receiver notices only n-1 cards have been registered and decides to fill in the blank, but that would still rely on the cooperation of the registrant, since the registrant will probably be asked which rares/bombs he remembers opening.

What we're trying to discuss here, is:
“why won't we allow a proxy in this scenario?”

and not:
“why don't the cheater find an easier way to cheat?”

This happened to me in a tournament I was TO for. The missing card was a rare (Whip of Erebos). We search the floor, tables, and checked the pools of the players in between the opening player and the player who received the pool. We never did find it.

There were no traders at the event, but I did have my trade folder with me, so I donated a Whip I had in there. I feel that entering Limited tournaments is not cheap these days, and players have a legitimate gripe if they are missing any card, but especially a rare.

Collusion is very difficult. The HJ should ask players to pass their pools along more than once, so there is no way for any player to know who will end up with their pool. I find it surprising that issuing a proxy is not allowed - if one of my players found himself short of a card, I think he would be extremely upset if that card could not be replaced.