Well, with the same reasoning, we don't need passwords or protection bits on files, since I can always take a piece of steel pipe and beat the owner, until he gives out the data, so why bother?
Blocking chown for general users is one level of several controls. Given the need, it is always possible to thwart an attack, with additional controls. And of course, given a set of controls, is is always possible to find an attack that will be successful. It all depends on the cost of the protection, the attack and of the data being protected.
Szabolcs