Fair enough. But it's a two per rest, short duration spell that against any decent enemy that has a negligible effect.
If you already wear heavy armor and cast AV, things look different though. You might even boost your deflection to untouchable levels. It's I think already a problem, that there are builds with so high deflection that they almost never get hit by certain enemies.
To make AV halfway balanced, it should give diminishing returns I think. That is, if your deflection is very low, it should boost it considerably by perhaps 50 points, and if your deflection is high-end, it should only get boosted by perhaps 15 points or so.
They could penalize the AV using the recovery speed of the armor worn and explain it away as some sort of interference effect.