Why do slows take a hit per person in party?

Even if it's a balancing bit why is it 10%, that means with a full group you have -50% just from the group not to mention magic mobs have what was it like 15% and 30% for rare totaling at 80% reduction. Uniques are 50% according to the tooltip so with a full group are slows just useless when bossing? Unless this isn't additive but the tooltip seems to imply it is...

Is there enough slows to warrant this?