Of course, further discussion is irrelevant if a hybrid spec can't perform both roles well, so let's look at each in turn. Since we're looking at this idea from a DPS perspective, we'll assume that you already have a full set of reforged cat gear, which you'd like to maintain.
The viability of a hybrid build
From a tanking perspective, bears are generally gemmed and reforged for dodge, so you'll have 3-4% less avoidance, a bit less armor without a tanking metagem, and probably a little fewer HP. No getting around that. However, what many people don't realize is that you'll have higher mastery and crit, which will result in a more powerful Savage Defense that is up slightly more often. Based on my calculations (assisted greatly by Tangedyn's spreadsheet), that means you'll take roughly 5% more damage than a fully kitted-out bear. That's it. When you consider that you won't be tanking 100% of the time, and thus your personal cooldowns have a greater uptime percentage, the difference becomes almost negligible. Oh, and threat? Virtually no change. Your extra DPS stats almost perfectly balances out not having Mangle glyphed.
Now, from a DPS perspective, the calculations are much easier. Using Mew, I simply unchecked all the marginal DPS talents I dropped (Primal Madness, Stampede, Predatory Strikes, and Feral Aggression) and ran the numbers. I'll spare you the gory details: A fully geared T11 cat with a perfect rotation loses about 1% DPS, plus Stampede's ability to Feral Charge->Ravage. That may or may not be useful, depending on the boss you're on. Overall, the DPS reduction is very, very minor.
To clarify: Many bosses in this tier have hitboxes that are large enough to enable you to Feral Charge while remaining in melee range. Via Stampede, Feral Charge then essentially becomes a DPS cooldown, as it lets you use a free Ravage and can boost your DPS a good 3-4% if used correctly. Some bosses don't let you charge at all, however, or make you run out to do it, which drops its value significantly.
The feral hybrid spec
The baseline hybrid spec picks up the four key bear talents (Thick Hide, Natural Reaction, Pulverize, and Infected Wounds) and leaves three free points that you can place yourself: probably some combination of Brutal Impact, Nurturing Instincts, and Perseverance. There are some other options as well, but that's a good start for now.
Glyph-wise, I'd definitely recommend slotting Frenzied Regeneration for raids; Maul, depending on the situation; and possibly Mangle, if you're really worried about bear threat. Your cat DPS will take a small additional hit, though.
For actual tanking pointers, let me direct you to Allison's excellent Feral Tanking 101 guide.
Other than that, your actual strategy doesn't change much. Simply DPS as normal, and if the tank goes down, go bear, taunt, pop a cooldown, and let your healers know. If you're a planned OT, then just do your thing, and use your speed in cat to get back to the boss when you can ASAP.
Advantages of a hybrid spec
Essentially, there are three situations when a hybrid spec comes in handy.
Disadvantages of a hybrid spec
The MT dies. Typically, a tank death means a wipe in 10-man raiding and frequently in 25-man as well. Raids don't carry a just-in-case tank, and usually by the time the MT gets a rez, enough of the raid is dead that it doesn't matter. Having a cat that can go bear can prevent this wholesale carnage, either as a temporary tank to let the MT get a rez or as a last-ditch save. As I mentioned last week, I have an addon that gives me a distinct audio cue when the tank dies. I've trained myself to automatically go bear when I hear that, as I'll typically be pretty high on the threat list anyway. Doing that has turned several potential wipes into kills for our guild.
A fight demands a temporary or semi-temporary OT. This means multiple bosses, adds, fights with stacking debuffs on the MT and fights with a tight DPS requirement. These types of fights are perfect scenarios for a hybrid spec, and there are lots of these in this tier. Cho'gall phase 3 is a perfect example: You have to have two tanks due to the Fury of Cho'gall debuff, but you might as well do some good DPS when you're not tanking, right? Pallies and DKs don't have to worry about rage, and warriors get to cheat with Vigilance, so you might as well cheat too by going cat. By the way, this isn't even considering heroic raids, which have many more opportunities for temporary tanks.
You replace the MT. If a tank leaves, you can step in and tank without any interruption. There's a very real risk that the raid will disband if people have to wait for you to go respec; with a hybrid spec, you can keep pulling. Of course, you can do this via dual spec, but I typically use my alternate spec for resto, which lets me fulfill any role the raid needs.
We are a unique snowflake. No other hybrid class has anything near this level of sustained ability at both specs. (Caster DPS hybrids can heal decently, but not for long.) We're definitely the only tank spec that can still bust out great DPS when not tanking. There are problems with this, though:
Blizzard devs don't like it, because they don't want classes that can do multiple roles to be able to do them simultaneously, as that devalues other players.
Non-druids don't like it, because it makes druids OP.
Bear-only players don't like it, because they want to tank, not half-tank and half-DPS, and they always get stuck as OT because they have the best non-tanking DPS.
Cat-only players don't like it, because they want to DPS, period.
All of these are valid points. However, I think Blizzard devs don't really mind hybrid speccing too much; they could have easily prevented this when they redesigned the talent tree, but they chose not to. The rest of it is player perception and player preference. Do you want to be a full-time bear, a full-time cat, or a little of both? Are your fellow raiders and raid leader able to handle the idea that you can perform both roles? Those are questions only you can answer.
Personally, I loved
being able to backstop one of our new tanks in a heroic 5-man or save the day in a raid while still doing competitive DPS (Kalon
calls it "herobear," which I like), but I can fully understand how that might put extra pressure on you if you don't
save the day. Heh.
Thoughts? Discuss in the comments!
What race should you choose for your druid? What happened to Tree of Life? How can you get started as a bear or cat in
Cataclysm? Shifting Perspectives has the answers!