There's lots of angst and concern going around in the WoW community over the introduction of dual specs. Here are my thoughts on who the winners and losers of this feature will be.
Big Winners: Healers and Tanks
Dual specs are pure and absolute win for healers and tanks. They get their main spec for raids, and then can do a variety of things with the second spec. They could go with DPS for farming and soloing, making that aspect of the game much less frustrating. Or run a PvP spec to participate in PvP. Or even run a second type of healer/tank spec, if absolutely crazy.
There's pretty much no downside for the healers and tanks. They get first choice of the healing or tanking gear, and can actually use their offsets.
Small Winners: "Pure" DPS Classes
For the "pure" DPS classes (rogues, mages, warlocks, hunters) dual specs are useful, but not game-changing. These classes will most probably end up using their second spec for PvP. You might see some of them using the second spec for Replenishment, depending on how popular those specs are. If Survival, Destruction, or Frost are popular in and of themselves, people may not bother.
I don't really see dual-specs changing the game significantly for pure DPS classes. Some of them might not even bother to purchase the option.
Big Losers: DPS Hybrids
DPS hybrids are going to take this hard. First, it's going to be expected that the second spec will be healing if you can heal, or tanking otherwise. No PvP specs for you!
Second, there still won't be enough healers/tanks, and lots of dps, so guilds will will pressure you into healing "just for now, until we recruit more healers." But they won't be able to recruit, or recruiting will suddenly become less urgent, so you'll see a lot of DPS hybrids get stuck healing.
Because the cost of switching will become so low, the norm will change such that switching to healing is the expected behavior, even if the player doesn't want to heal. After all, why spend all that time and effort trying to recruit scarce healers when one of your DPS hybrids can switch for zero cost?
Finally, because the hybrid still considers themselves as DPS, they'll won't get primary consideration on healing loot, as the healer mains will have that. However, adding insult to injury, I predict that many guilds will start giving the pure DPS classes priority on DPS gear. This will be justified on the grounds that the pure classes will use the gear 100% of the time, while the DPS hybrid spends a lot of their time healing. So obviously the pures will receive priority "for the good of the guild as they will get the most use out of the gear."
I think the healers and tanks will be very happy. The pure DPS classes won't really see much change. But I think there will be lots of drama surrounding the DPS hybrids.