I found that at 7% haste your DB does one additional attack and when you get up to 18% haste it does two and for the next extra DB attack you have to have BL/Hero up with RF or FF and with all three it does 13 attacks in total. And I know that haste is useful for other things like our focus regen and the amount of auto shots we do. There for with 2pc I prioritize haste for more focus for a few extra Arcane Shots. Seems to work for some fights.