unlike the previous version PD has a positive effect on accuracy, you still need lots of wpf to be accurate but some pd above the requirement on bow helps accuracy. only thing horse does is decrease accuracy while moving and has the damage penalty, all other mechanics are the same.
just to help you out a bit for a horsearcher you want 3-5pd 4-5horsearchery 8-10wm and 4-6riding
so while pd is good for accuracy, on a horsearcher the horsearchery skill is most important so you will most likely not have enough stat points to go for very much pd