Someone pointed out that the fact that TrueCrypt allows a hidden system gives an attacker the incentive to torture you until you reveal the secondary hidden one.

<shrug> If you're really paranoid you can implement a nesting-doll system with as many levels of depth as you want.

But that argument applies just as well to anything at all: the existence of flash drives (or, say, treasure chests filled with gold) gives the attacker an incentive to torture you until you reveal the location of your hidden data (or treasure).