If all goes according to plan, Warlocks will get the Replenishment buff in Destruction. Because Blizzard considers Replenishment a mandatory raid buff, this hopefully means deep Destruction will be more than viable as a raiding spec. Because a lot of changes are coming to the Warlock talent trees and abilities in Patch 3.1 -- if Blizzard follows through with their plan -- it's too early to predict what builds will be viable as the two choices for builds. On the other hand, we can take a look at the playing style options and what this means for players and raids.
It still depends on exactly how Blizzard will implement it, but it looks like we'll be able to change our specs between encounters. Inscribers will be able to produce a portable Lexicon of Power that's required to do this, but details are still sketchy. At any rate, it's quite likely that we'll have the opportunity to change spec depending on the raid boss we're fighting. For very short fights, we can opt to go all out nuke with Destruction, while on longer fights or those with a lot of raid damage, Affliction is a good option.
Speccing deep Affliction can provide the raid with Malediction, while deep Destruction will provide Replenishment from Improved Soul Leech. It remains to be seen how deep Demonology will be made raid worthy as 0/ 41/ 30 is still the deepest the most popular raiding specs go into it. Demonic Pact doesn't seem to be compelling enough to go deep Demonology, even for raid utility. Then again, by the same token, 0/ 31/ 40 is the deepest a lot of raiders go into Destruction, but Replenishment might be a good excuse to go deep enough for even Chaos Bolt. Hopefully dual specs and the changes in 3.1 will see deeper builds.
The other cool thing about dual specs is it gives us the flexibility to spec for specialized roles without gimping ourselves for the rest of the raid. An old example would be Leotheras the Blind in Serpentshrine Cavern. There isn't a Wrath boss specifically designed to be tanked by a Warlock, but as some players have proven, very specialized roles can prove to be effective in some fights. Obviously, tank spec won't be a common choice for most -- and Ghostcrawler has said it won't be an option in the future -- but you get the idea.
Dual specs will also allow us to adjust according to raid make-up. If we have Moonkins or Shadow Priests in the raid, we can opt to skip Suppression and Cataclysm, allowing us to put our focus elsewhere. This gives us a modicum of flexibility or additional DPS by foregoing redundant +Hit. Theoretically, we can bring our 5-man builds to 25-man raids depending on our group make-up. It's a minor thing, but it squeezes out every bit of DPS we can muster.
The most popular Warlock build for PvP today is the new incarnation of SL/SL, a 54/ 17/ 0 build that picks up Soul Link and Master Summoner. Players who enjoy the Affliction play style, which I confess to, can run the 53/ 0/ 18 PvE Affliction build and spec to PvP Affliction when necessary. Ironically, Demonology has fallen by the wayside as Resilience and healer survivability has increased over the past weeks. This has reduced or even eliminated the need for the survivability offered by Metamorphosis. Soul Link coupled with the increased health of demons has greatly increased Warlock survivability in PvP.
I have to point out that there are a lot of Warlock changes coming in Patch 3.1 that all of these builds will be slightly -- or greatly -- different once the patch rolls out. Demonology, in particular, is getting a second look and will probably be receiving an overhaul considering how underutilized it is right now. Gimmick fights with dragons aside, deep Demonology isn't an incredibly popular raiding or even PvP tree. In the current environment, it's unlikely that players will be speccing deep Demonology as their second spec even for PvP. Does this mean I'm calling for Demonology buffs? Hell yes.
That said, players can have either different styles of PvE builds as their two specs, one spec for PvE and another for PvP. In fact, with the kind of flexibility dual spec allows, we're not constrained to the archetypal roles that hybrid classes fill. While some hybrids will almost necessarily have a healer or tank spec tucked away as their other spec simply to be viable in groups, we won't need to. It's a blessing in disguise. Again, we kill things. As long as we kill things well, it doesn't matter. The other spec is something we can do purely for our own pleasure, even something as simple as the frivolous desire to solo farm. We have so many glyphs we can choose a second spec purely for the sake of having different glyphs.
Other classes won't have that luxury. Paladins, for example, will be expected to have either a tank or healer spec. I assure you that Paladins who don't spec for either (like some stupid Ret PvP / Ret PvE combo, maybe) will be wildly ostracized for their inability to be "more useful" in groups. We have no such expectations. Shamans who spec Enhancement and Elemental DPS are going to be looked at with scorn. I mean, really.
As Warlocks, we have no such dilemma. In fact, the likeliest common dual spec options for most of us will be one for PvE and another for PvP. It's that simple. Those who only raid exclusively can maintain one optimal raiding spec and another for AoE farming to earn money in between raids. As exciting as dual specs may be for hybrids, it's emancipating for Warlocks. Imagine, we get to kill things in whatever way we see fit. If we get bored with Affliction, we simply toggle to, say, Destruction at our discretion. I promise you, when dual specs roll around, we'll have one hell of a good time.