It is already common knowledge that all 3 hunter specs are getting some major balancing done to them. So the question I have asked myself (and probably a lot of hunters have too) is, what spec will I be raiding with once 4.0.6 is live. In the past I've always gone with whatever spec is considered the highest DPS, but with Cataclysm that has changed. Now we need to select specs also on the basis of what it brings to the table other than its DPS numbers.
As much as Blizzard have tride to balance the specs to be even in DPS, it was evident early on during this expansion, that it was not the case. Survival has outdpsed the other specs, like Usain Bolt wins the 100m - it wasn't even close. So Blizzard has needed to make some changes. Survival was always going to get nerfed, how heavy will remain to be seen. Marksman and Beastmaster will get buffed as expected, and at last check numbers between all 3 specs could be very close.
So let's say that Blizzard succeed in balancing all 3 specs, the choice for which spec to select will not be as easy as pick what your favorite is. In Wrath it would have been easy, but not in Cataclysm. Why? Because all 3 specs have aspects to consider that we didn't have to in the past, either because they were the same across all specs or because certain spec mechanics have changed since Wrath.
In Wrath AoE was effectively the same for all 3 specs, now not so much. Survival has and probably will continue to have significantly better AoE dps then the other 2 specs. This is an important point to consider when certain Cataclysm raids require some pretty heavy AoE fire power. Magmaw and Maloriak spring to mind. Survival has talents that significantly improve AoE DPS, and lets not forget talents that improve controlling multiple mobs through use of traps - extremely useful on those fights again.
Now consider Beastmastery. On a straight forward fight a hunter's pet will usually do about 40-45% of the hunter's damage, that means almost half his damage. The Beastmaster's primary attack is Kill Command which requires the pet to be in melee range to deal damage. That's all good and well on a tank and spank, but on anything that requires a lot of target switching, say Omnotron Defense System, you're going to run into a bit of a problem. And on Magmaw, where you already are going to be pre-occupied with the lava parasites, you may not necessarily have the opportunity to take care of your pet, meaning it will most likely die.
Marksman doesn't have these pet issues, and also has a useful talent for multi-shot (albeit one that is hardly ever taken), but it will depend on how high (or low) its AoE DPS is, for it to be a universally successful spec. With its rebalanced DPS it may be fine for a average boss fight, but if it struggles to keep up with Survival's AoE numbers, chances are it will not be the spec of choice for raiders and may not even be taken as an offspec to allow for BM's exotic pets.
So what will people go with, which spec will they choose? Well, here's what I think will happen. Hardcore raiders will go with whatever the top spec is and have Survival as an offspec (provided its not the top spec). Your casual hunter types will just go with whatever spec they like as they have in the past. In the event that all regular DPS does happen to be balanced among all specs, I expect most raiding hunters sticking with Survival as main spec (because of its AoE capabilities) and Beastmaster as offspec, for extra pet options. At least, that' what I'll be doing.