As a hunter (well for my main character, in the few days before crunch time, my pet is a big part of the arsenal available to me. I'm quite happy, most of the time, with my bear. Extra armour and extra hit points a nice tank maketh, and if the damage is a little lower per second, that's cancelled out by some durability on those times when I make a mistake and send the poor beast in to a stickier situation than expected (like when I get hit by a load of big critters who all respawn on my head in the middle of an over-level fight).
But I was tempted by the thought of big scorpid venom after being hit by one, so I swapped pets, briefly. Ouch is the only description I can think of. The venom is great in PvP I'm sure, but not so useful vs. critters, especially critters with poison themselves. The absence of growl and the ability to attract and hold aggro meant I was suddenly in the midst of fighting all the time. The fact that, even after the scorpid levelled I couldn't teach it growl (or similar) was the final nail in the poor scorpion's coffin. For PvE play I want that tank bank and doing what tanks do best - holding aggro. If I, as I do, sometimes tip over the edge and pull them off him by mistake, so be it - for most things I'm fighting at around my level, it's half dead or more before it disengages. There is still time for a stun and an arcane shot for a bit more damage before contact. It leaves the daggers I use as a nice back up to kill the almost-dead critter as it arrives.
The moral of this tale - for both Hunters and Warlocks I guess - think about your pets/summons and what you use them for. Piling out extra damage is nice, but do you want a tank more? If you're hunting in a group, does that change? (How much would the group tank like a pet that slows the attacks on him from ancillary targets, I wonder?) Pets are an important part of your arsenal - one I use all the time and I'm sure most hunters and warlocks do too. Thinking about how to use them is surely smart?