I'll keep this short and sweet due to character limits. Reasons to start a battle with Verthunder/aero instead of Jolt II:

Jolt II has less potency than all Ver spells.
Jolt II offers less Mana Gauge in total than all Ver spells.
Acceleration procs Verfire/stone off of Verthunder/aero, not Jolt II.

Debates to use Jolt II instead of Verthunder/aero:
Longer cast time. Matters in battle, but not before battle. Moot argument.
Costs more MP. How much more? Not enough to matter.
Timing. Easily the biggest argument. Remedy? Diversion. Tank can grab aggro from you if you happen to cast before they pull easier with Diversion lessening enmity gain.
My DPS doesn't matter in this dungeon. Legit argument, if it's true. Endgame content is not this forgiving, especially if you queue via Duty Finder.

I used to open with Jolt until I heard someone mention opening with Ver spell is better because cast time doesn't matter pre-battle, and I cannot argue that. The truth is undeniable.