Using an oGCD (Lightspeed) to make every AoE heal to land 2.5 seconds earlier
is a boost though. It increases healing rate in the moment it matters, even if the AST has to pay for it later by not landing any GCD heals for 5 seconds. If a WHM or a SCH uses swiftcast they only gain that burst for one heal, then they have to wait for the global cooldown + cast time of the next heal. AST can keep throwing instant heals until danger is over, essentially gaining one extra heal on top of the usual rate of heals. At the same time they are saving some MP. While Lightspeed may look like a Swiftcast, it's effect lasts longer and that's where the cooldown's strength lies.
Here's a demonstration of what I mean (and my amazing Paint skills

) using some example spells:
The red part is where SCH could use Succor over and over but I think most agree that Succor spam is not very efficient. The yellow part is on the left side for clarity but healers would likely start pre-casting the first heals so they land right after damage has been taken. For SCH Deployment would be used before the hit and the rest right after. For AST you could replace the A. Helios with CU if it's up, and then cast A. Helios in place of the first Helios.
EDIT: Changed pic a bit because I play SCH so rarely I forgot a few spells

