But keeping the veil in such a high or low state through couriers and tahren is not really tenable. So you do it selectively when you anticipate you'll be fighting soon after.

Thick veil, magic will fail, spells, talismans, potions etc.
Thin veil, the occassional double cast. The spell will arc to someone else you are fighting. This can be very beneficial. i.e. two novas for the cost/lag of one. IT also changes how much save spell a rager gets from spellbane, but not sure if it impacts how easily you actually bane spells.

I don't believe the efficacy of a spell, is impacted by the veil. So the damage should remain constant. Not sure if spells are easier to save against in general when the veil is thick, harder to save against when thin.