And that's the key point. 550 points for Kroak sitting on his vortex doing nothing but summon means that you're playing roughly 1500 points to your opponent's 2000. There's the value of his unbinding rolls, but that's a very tough thing to value, since it's entirely dependant on what your opponent has. Against Tzeentch, unbinding is extremely high value. Against Khorne, it's worthless. Casting spells on the other hand is always worthwhile since it's not dependant upon your opponent's army. Tossing out a Starlight onto a key unit is always useful.
Furthermore sticking your Slaan on a Vortex means he's most likely stuck out in the open for everyone to have a good old shot at, whereas keeping him on the ground offers far more opportunities for hiding. Even Kroak isn't invulnerable, and if you lose your Slaan and Astrolith Bearer then any summoning points you gathered are entirely useless.
I'll reserve judgement on summoning until I get some games under my belt. Currently I'm much more worried about Slaanesh summoning than Seraphon; the thought of my Carnosaurs being batteries fuelling Slaanesh summoning has me quite concerned. But for Seraphon, I think summoning will support our tactics but not replace them.