Of course. YOu are 100% right. But what is different here is my ass is on the line. Thief should plan on his ass being kicked if he is giving out his real name and address like I am. MtGox and MyBitcoin could stiff you any second, and your bitcoins could disappear if they get hacked (as has already happened to some).
if you waited five years and stole from just one particularly wealthy address, your deniability would be quite plausible and even, very likely, convincing in context to many people. who could say it was you who stole anything, rather than someone who illicitly gained access to the customer's sheet of paper in the intervening five years?
i actually trust you, and i doubt i could name five other members of this forum i would say that about. but sadly i would not be able to recommend a service like this.
Just curious, would you also say the same for MyBitcoin.com?
absolutely. indeed, given recent events, i trust them far less than i trust you. there's no good way for them to prove that the thefts that recently occurred at their site weren't at their own instigation or for their own benefit, just as you wouldn't be able to prove something similar the first time anyone anonymously stole money from one of your customers' addresses.
for similar reasons, the business isn't robust against that sort of bad publicity. if i were a competitor, all i'd have to do is buy one of your sheets, transfer 1000 btc to it, transfer that sum away, and then claim you did it. who's to know? getting rid of trust solves that problem too.