After foolishly stating that I don't see any use for dual-spec for a mage, I promptly ended up in a Naxxramas raid with a deep frost mage, who was extremely useful at Gluth, and much less so at Sapphiron. Now there we could have used dual-spec for mages. :) But the pros and cons of dual-spec seem to be a hot topic, so I'm dedicating this thread to it.
The whole discussion starts with the eternal trinity: tanks, healers, damage dealers (I'll use the short form "dps" for them in the rest of the post). The fundamental problem is that the trinity only works together, that is in well-coordinated group play. If you are alone, neither "tanking" nor "healing" will achieve anything. So both in solo play and in un-coordinated PvP play, dps is king. The more damage you deal, the faster you kill mobs, and the faster you gain xp and levels, or get your daily kill quest done. The damage mitigation part of tanking might be helpful, but the taunting part of tanking doesn't do anything in solo or PvP, and ultimately you'll need to deal damage to win. Same thing for healers, healing is useful, but you can't kill a mob or other player with it, unless you cooperate with other players.
So Blizzard, from the start, offered at least one talent tree for every class which was dedicated to dealing damage. So a tank would have at least one tanking talent tree, and at least one dps talent tree. A healer would have at least one healing talent tree, and at least one dps talent tree. So far, so good. But switching talents isn't all that easy in World of Warcraft. You need to pay to unlearn your talents, manually choose all the talents of your alternate talent build, and before a recent patch even pay for re-training higher spell ranks of talent spells. Consequently most people didn't switch from one talent build to another all that often. A short guide on how to play a priest in WoW was: "solo to the level cap in shadow spec, then switch to holy / discipline and heal raids". Obviously not a perfect solution, because maybe that poor priest would have liked to group from time to time while leveling up, or would have liked to do some PvP between raids. There were stories of priests getting kicked from raiding guilds for the sin of respeccing to shadow.
The promised dual-spec feature is trying to solve that problem. We don't know details yet, but it is expected that instead of having just one talent build, you'll have two, with an easy way to switch between them, and no cost involved. So if you want to solo and PvP with one spec, and do heroics and raids with another, you can do so.
Nevertheless the introduction of dual-spec is not without problems with regards to class balance. A pure dps class, like a mage, might enjoy the possibility to switch from frost to fire, or from a single-target raid build to an AoE build. But if he is in a group desperately looking for a healer or tank, he still can't switch to fulfill those roles. A two-role hybrid, like priest or warrior, has more utility from dual-spec, being able to switch from dps to healing/tanking and back. And the three-role hybrids like paladins or druids will have to choose two of those roles for their two specs. The ability to switch roles comes with a price tag attached: The necessity to switch gear as well. The mage might well wear the same gear for his solo and his raid spec, but somebody switching from dps to healing or tanking will have to carry a second set of armor around.
I hear different information on how easy it will be to switch from one spec to another. There are other games, like Final Fantasy XI, where you need to be in a city to switch roles. But some people say Blizzard wants to make switching so easy, you could do it in the middle of a raid. Trash on the way to the boss too easy? Mow it down faster by switching your second tank and third healer to their dps builds. I'm not so sure whether that is such a good idea, because it could easily backfire in the serious raiding guilds: Instead of gaining the liberty to have one build for doing other things than raiding, you might be required to have two raid builds, for different situations and encounters. Priest starts with discipline build, buffs the raid with the spirit buff, then switches to holy build for general raid healing, and back to discipline for single-target healing. That is actually something I might do voluntarily, as I'm not a big fan of shadow priesting, but I wouldn't like if guilds started to require that for raid participation, because you're back to square one of the original problem.
The other problem of dual-specs I frequently hear mentioned is that it removes the meaning of choice. I'm kind of proud that I've always leveled my priest as holy, and my warrior as protection spec, because it was a choice I made to be able to join more groups while leveling. With dual-spec that choice is gone, and my warrior who is currently switching just gear from dps to tanking will in future switch talents as well. People don't need to choose their talent builds to be viable for different situations, they'll just switch to a different tree whenever necessary. With two builds you can cover a large majority of your various talents, thus there is less having to refrain from doing one thing in order to be able to achieve the other. Ultimately it removes a feeling of identity from your character. You're not the "holy priest" any more, or the wicked "shadow priest", you're both, and thus feel like neither. Probably more a problem for the few role-players out there, who'll all suddenly have to invent a story to explain their split personality.
So overall I think most people will welcome the possibility to dual-spec, in spite of the risk that it makes World of Warcraft more easy, or more bland. I hope it will be done in a way which enables people to switch with ease between different game activities. More options and variety is usually more fun, and for Blizzard a longer subscription. What do you think, is dual-spec a win-win proposition?