Ok, I'm currently leveling Tyrana now with WotLK. While leveling again, I've realized a thing or two about Volley. Some you might consider obvious, some perhaps not.
My Steady Shot costs ~130 mana. If I get lucky with crits, I have to use 2-3 of them to get a similar level mob down. If I'm unlucky it can take up to 8, but mostly 5-6 does the trick. That's... around 650 mana spent on one mob.
I've started to use Volley while grinding. One of the reasons are that it only consumes 1 ammo, but damages mobs within the radius 6 times. Another is that my steady and auto shots are at around 350-450, and my Volley hits for ~400 unresisted, and crits for ~900 unbuffed (over 1k with BW popped). Yet another reason is that you get 6 shots for 560 mana, rather than 6 shots for 780 mana. Not to menntion the scaling with mobs within the vincinity.
With Steady Shot and Arcane Shot and Multi-Shot (the two latter being slightly less mana efficient) you still have to use 5-6 new shots when the first mob is down, landing you at up to 1.5k mana lost for two mobs. While you can get lucky (with crits) and escape with only a minor mana loss, Volley is a far better option for these situations.
It might seem obvious to everyone that Volley is the more efficient for 2-mob-pulls or greater, but I'd like to argue that it's the most efficient even on single pulls.
As I've said, you only spend 1 ammo, this saves a lot of money if you continiously use Volley for everything.
You can use the ammo you save to regain mana with AotV (and with a clean consience, since you would've used it for something much more inefficient if you weren't using Volley) since Ammo is still cheaper than water.
Now, for the obvious part: You're restricted to one pet. This won't work on any other pet than Gorilla. Not even Carrion Birds if my estimates are correct. However, I was under the impression that most hunters play with Gorilla now, so I though I'd share!
Any comments or input on this? Is this a playstyle you'd use?