Lightspeed is a very useful skill. I use it a lot and I'm glad to see its cooldown being reduced, though I'm not sure if it really needed to be reduced. But hey, buffs are buffs.

I primarily use Lightspeed as a means to speed-resurrect allies, especially when Swiftcast is on cooldown. Combined with an Arrow card buff, you can cast Ascend at a ridiculous rate. I also occasionally use it as a secondary swiftcast to double Helios in a pinch or as a means to cast AoE heals while moving. This can be extremely useful in certain situations where you are forced to move for an extended period of time to avoid getting killed by mechanics, but are also forced to heal to prevent your party from dying. An example of this would be NidhoggEX if your party is low on health heading into the next Akh Morn and you need to get to Waymark A while also healing the party.

To be honest, I had completely forgotten that it reduced MP cost. In any situation where I'm having to raise a lot of people, and am therefore using a lot of MP (like learning parties), I run hard MP sustain, prioritizing Spear and Ewer cards over offensive cards. So MP is not a concern for me when I'm playing AST.