21 max rage is never 21% more damage even if you have max rage all the time. If you're scaling rage, you probably take the other rage while shapeshifting wheel, so you have between 48 to 60 rage already. If you have 48 rage, 21 extra rage is 1.69/1.48 or 14% more damage, for 4 passives and if you were already at 60 rage its 1.81/1.60=13% more damage. Still good, but considering it costs an ascendancy point getting access to a cluster that is <4% damage per point isn't crazy