Virus is useful if you fight enemies that telegraph their big hits or otherwise have a set rotation of moves they use. It's very useful in the Titan fight for reducing the damage to your tank from Mountain Buster as 10% off a 4k hit is a significant difference.

Surecast has started to become more useful in endgame now that there are bosses that have unavoidable AOE that can interrupt your casting.

Swiftcast is absolutely amazing and you should pretty much go straight for it ASAP. Being able to cast a Medica while running or instantly Raise someone is invaluable.

Thunder is kinda useless once you get Stone II, honestly. Why cast a DoT that equates to 150 potency when you can just cast a spell that does 170 up front? Aero and Aero II are great though, their resulting potency is much higher than Thunder's.