I still use both multi and volley. I use multi whenever I can, it crits so much it's not funny. Volley got used a bit in certain circumstances. In combination with explosive trap, pet on aggressive and instances about 3-5 levels below you, you can AOE grind semi-effectively. You need to be careful of the numbers you pull though. Used it a bit in Uldaman and to in other instances with multi-pulls or mass targets.
As for the spec you choose in PvE... don't worry about it. Play all 3 if you like. BM may be 'easy mode' PvE, I don't know, never played it. I did play 70 levels of MM spec mostly on a PvP server then switched to a PvE server (doing PvP while trying to level is a time drain!) I never had a single problem that MM spec couldn't do PvE. There are some gotcha's with MM though:
• When you get a new rank of a powerful shot, be ready for your pet to lose aggro if you light that puppy off too soon. BM pet develops aggro much faster than any other spec so they can lay into the target harder, sooner. MM, you'll be on autoshot for a bit unless you follow a long lead shot & duty rotation:
Lay Trap, Mark Target, wait until 10 sec left on trap CD, Send Pet, Mend Pet, Let Pet Growl at least 1 time, Aimed Shot, Auto, Pet Growl #2, Auto>Steady until target is dead.
It's a great solo routine and if you learn good pet control, can deal with about 3-4 targets at once, 4-5 if you pull one back into your trap.
• You need to MASTER your group skills. It's being propagated by ignorants that MM spec is useless, has no merit and no functions in a group. I got this a lot before I ding'd 70. The only way to prove these folks wrong is that you have to be on top of your game.
As you level, you won't have any of the advanced shots, but don't worry about that, no other spec has their super stuff either. What you do have to do is learn to leverage what you do have to the best use of the group. This is true for all specs, but MM especially, since early on you are really not much more than an artillery piece. Your pet doesn't do as much damage and you do the bulk of it.
That makes you PERFECT for pulling aggro off the healer or casters. Hang with them, and be their guardian angel if things go south. Use your pet if need be to help you defend them. Generate aggro like a mad man and run the mobs back to the tank, once the tank has exceeded the aggro rating of the casters/healer, you can FD and they will turn on the tank while you go bandage yourself up.
Think GROUP, practice for it with your guild buddies.
Read the stickies at the top of the hunter forum and plan how you will practice, after all, the first 69 levels are just practice for 70. Don't just complete quests and grind, do it with a learning purpose in teaching yourself how to best play your toon. Took me 62 levels and two years to learn that lesson. I wish I had learned it much sooner, I'd probably be much better as a player by now.