Yes, there is utility for those points after you've unlocked all souls. The next anniversary update will introduce passive stat bonuses and new ex skills for your soul. They'll cost Master Points (MP). You gain MP through exchanging soul points (and holy soul points) at a rate of 100 per 1 MP. You also gain MP through gaining experience with a lv 20 soul (50k exp for 1 MP, I think?).
A new ex skill costs 15 MP. For the passives, I think it's 1 MP per level of a passive (max of 3 levels per passive), with a total cap of 20 levels overall per soul. But I'm not 100% sure on that.