Do you think that the Ranger class (if there is one) in Diablo III should use "mana" or another system like the barbarian's? Since mana is now gained from the orbs.. what do you do after you spend all of your mana on your arrow attacks? Run in to the swarm of monsters and collect some mana orbs from some of the monsters you killed.. and then run back and shoot some more arrows? You would get mauled to death! The same goes with health, if another ranger or long distance caster monster is attacking you from far away and you need more health , you have to run up to them to get health orbs from the monsters you just killed.
This could be solved by using the same energy system as the barbarian (but then you still have the health orb problem) or.. by introducing a sword/bow class, so that you cas start off with a ton of arrows and then slash through with your sword to get to the orbs. (also great for when you run out of arrows)
They never said that mana was exclusively gained from orbs. I'm sure there will be reasonable regeneration rates, there's potions, of course, and there will probably be ways of gaining back mana through attacks and skills, but sure, I wouldn't mind a new and interesting form of energy collection for a ranged class (assuming there is one, though there probably will be). I wouldn't want it to be a copy or even vaguely like the Barbarian's, though, and they just change the skin. If they were going to do that, I'd say just go with mana again.