Running someone low on mana is a tactic, and ought to remain one in situations where you can't run them out of hp until you've run them out of mana.

For example, a paladin or shaman is a lot easier to kill once their mana has gone, or if they need to conserve it. I'd they could replenish it at 100 mana a pop it would make them even tougher when that's not needed.