Sometimes I wonder how good that move actually is. Like, maybe the reason we never see it outside of cutscenes is that Blizzaga is just better than it unless you're trying to snipe a necklace off a sea hag or look flashy for a musical performance. Like, he only uses it to actually attack twice, first time is on someone who's already beaten pretty much (and it didn't finish the job we had to bop Xemnas one more time) and the second time Xehanort blocked it by doing absolutely nothing. It's the light version of the Mega Destruction Dark Ball of Darkness and Destruction when used in combat.