Titan Egi for example takes 3+seconds [Summon + Egi Assault + Laggy Casting Animation], so its not the greatest for emergencies either. Having VerWard give a large Duration like 30 seconds, [and maybe also giving accel two charges because why not] is so that you can use it ahead of time.
It would consume Acceleration, so you would only get the mana from the first use. Losing 2 mana, won't push us back much compared to losing 6-9 mana everytime we use it, with infrequent use it probably wouldn't push us back at all. Giving it to us on GCD also allows us to use it during long transitions like we currently can now with vercure or to proc dualcast without overheal. [A seperate skill could be introduced instead of acceleration, but I think for the sake of making Accel more interesting and fighting button bloat it would fit well as is.]
Planning ahead in fights is where you would feel it. BLM/SMN can't just pop their OGCDs Shields either, an open weave window is necessary. Popping it the earliest you can will ensure its up and not interfereing with your rotation. [SMN really can't use their shield either because of the loss of Auto Attack potency, loss of the Egi Assault and the Ruin IV proc loss zzz. So they really need something as well.]