Per user salt means that an attacker who has stolen the password db can't crack one password and unlock 100 accounts because they all have the same password has

Per user salt means that an attacker who has stolen the password db can't crack one password and unlock 100 accounts because they all have the same password hash. That is the only point of a salt it serves no other purpose. I wouldn't say no other purpose. A single common salt can at least protect you from the guy who has a pre-computed lookup table of dictionary words hashed with a standard, unsalted function. (That is, it prevents a zero-computation attack.) Admittedly that doesn't gain you much these days, but it's not "zero benefit".