Khorne's works on units destroyed so you could cripple a unit and then disengage from it if you have the upper hand to deny them the summoning points. Between Sage's Staff and Astroliths, we are actually capable of shutting down Tzentch casting. Sure, we're not going to keep up either of these things for a whole battle, but we could starve them of points long enough to leave them unable to fight back properly for at least one round. In objective-based games it might just be enough, and it's something they can't do to us without shutting down our summoning entirely.
This isn't just on an us-fighting-them thing either, by the way; a Khorne army against a particularly tanky army (like Disposessed, maybe?) might have problem killing enough units to get something they plan to summon onto the field in time. We won't have that problem. Tzeentch armies against armies with good unbinding abilities (again, Disposessed's heroes spring to mind) might not manage to get enough spells off, especially if their opponent isn't casting any (I feel like Disposessed aren't going to have too many problems with daemon summoning). We won't have that problem. A Death player might need those Command points elsewhere to quickly deal with a particularly tricky opponent like Allarielle. We won't have that problem. And given that all we have to do is sacrifice spell casting, as a faction with a lot of access to a lot of spells per turn, the only problem we do have is based on how much we really need to be casting spells on a per-turn basis. And assuming an average 2 from an Astrolith, you only need to forgo one spell to set up 10 skinks every turn, which still lets your Slann cast two (plus Cogs and Balewind bonuses) every turn.
