Lightspeed as an "emergency healing tool" is mainly for allowing you to cast Benefic II/Aspected Helios at less cost. The being instant-cast/faster-cast is less for "emergency" and more for movement.

The Ewer change combined with the Lightspeed cost reduction should help sustainability issues, but AST still has a bit of a throughput issue. Lightspeed helps in letting you use your bigger heals more often (and being more reliable of a cost reduction than the old Ewer), but I'm not quite sure if it's going to be enough yet.