Could be that John Palmer feels that in the beginning the cell density is low and not too many yeast cells, if any, would be damaged by shear stress. But after 12 hours the cell density is high enough that shear stress becomes a problem. Right or wrong, that logic is why I now put my starter on a stir plate the night before brew day and when I get up the next day I take it off and begin the shake and stir method. I figure that a stir plate overnight is better than nothing, which is what would be happening when I'm sleeping. Or it could be that I'm just rationalizing because I built my stir plate a couple of months before @denny said stir plates are bad!