Everyone's already said it already, but BM is key. the added health, armor and dmg provided by your pet helps him last longer in battle, take more damage, but what no one ever seems to realize and what a huge factor it becomes, is the 2% health regen from spirit bond that not only you get, but your pet gets as well. doesn't sound like much, but lets put it in perspective. i'm a  level 70 BM hunter and when i switched to MM, the first thing i noticed was constantly having to stop after fighting mobs in order to get my health back up.

as a 70 with 11,200 health, i gain roughly 225 health every 10 seconds, or 1350 health a minute while my pet is alive. i don't think anyone actually ever mentions this as a high point of being a BM spec, but in any serious raid, how often do you get to a boss in an instance that requirea a solid 2-3 minutes of solid pummeling to take down. well, i'm usually the last one standing when i'm around a MM hunter cause i might have regenerated a good 4000 health in that 3 minutes, this saves me from having to heal my pet as often, my priest from healing me as often, or having to stop and bandage in the middle of a fight. i can't say enough about having spirit bond. plus, your health doesn't typically regen during an active fight like your mana will(although you can't be casting), so having that extra 2% goes a real long way in a lot of instances