I don't see this being really useful to "Buff Dispenser" kind of casters, as you usually buff the whole party before any fight and if you have to refresh some buffs, it's nowhere near enough to exhaust you. Now, for all the "Blast it until it's dead" casters such as Sorcerer, Warmage or Warlock, it would definitely be a welcome addition, especially considering the later infinite spells. Though to be fair, having played a Warlock up to level 7 before taking a break : 14 Con and Endurance is already quite enough to go through many fights before even reaching exhaustion.
Hell, the few times I did get exhausted were when I was going from the Mist Camp back to Vallaki and found myself blasting stuff for longer than expected (Dire Wolf and Worgs are nasty...). In all dungeoning instances, I'd never reach exhaustion before the party needed to rest to refresh buffs.