The problem with this line of thought is there is almost no situation in the game that simply boils down to "just spam Helios some more" (which is where AST would shine over SCH anyway). By the time SCH runs out of CDs the AOE heavy hitters are done. If you need to spam AOE heals for a longer time it's usually because your co-healer is dead, and if your co-healer is dead during an AOE-heavy phase it's GG, especially on Nocturnal AST.

I personally find myself thinking that Lightspeed lasts too long more often than not. I'd prefer it to work like Rapid Fire (2.5s cast time reduction, decreased GCD, but limited number of uses) and it would easily fill that Assize/Indom niche.