I see it as a kind of side-channel defense; take the set of passwords which it is possible to crack quickly and use the work factor to even it out so that the t

I see it as a kind of side-channel defense; take the set of passwords which it is possible to crack quickly and use the work factor to even it out so that the total time to crack a password remains relatively constant. Of course it's a good point that the challenge then becomes determining what that set of passwords is. And not having run the numbers, I can't be sure how much room for improvement there is.