Xykon has already burned two Meteor Swarms on what amounts to one battle (forgot to cast defensively for the second, or was interruped by a readied action - either way, spell expended). Even a Sorcerer only gets so many 9th level spell slots. Xykon, however, doesn't need to deal with four encounters per day; at the end of the current scene, he can simply Teleport / Plane Shift out and get some rest. He doesn't have a tight timeline he needs to stick with, no particular reason to conserve.
He's spell dumping (although not as quickly as some) because he can.
By the nature of the class, everyone who takes it knows they're doomed eventually. Anyone who continues to cast spells from their pool is going to have some fatalisim in their outlook; they have goals that take precedence. When you have a goal that takes precedence over your life anyway, when something threatens that, you deal with the threat pretty much regardless of cost. First Wail of the Banshee didn't do the job? Time for another (or a different mass lose effect).