For this to be significant you first need a) to register on that site b) to explicitly logout and clear all the cookies for that domain(otherwise they can track

For this to be significant you first need a) to register on that site b) to explicitly logout and clear all the cookies for that domain(otherwise they can track you anyway). How many people does this? Of course, disabling auto-fill option is a smart thing to do regardless of this, I'm just saying that this is not affecting the most of the population.