I've been following Blizzard's comments about the dual-spec system that is probably coming after WotLK. I am a little worried that the system they are designing solves a different problem than the one many players want to be solved.
Let's start at the beginning. There are three "facets" to WoW: Group PvE, solo play in the world, and Group PvP. Many individual players want to able to participate in all three facets without going to a capital city and respeccing. The system Blizzard is designing theoretically allows you to do this, but it also allows you to change spec *within* a facet, as well. As Ghostcrawler puts it, "Since dps is your primary concern most of the time, this gives you a way to have say a trash spec and a boss spec, or a cc spec and pewpew spec without hurting your performance on either. "
Effectively, there are two goals:
Goal 1 - Participate in all 3 facets
Goal 2 - Have flexibility within a single facet
The problem here is that there is a conflict between individual and group. The individual wants to be able to participate in all three facets. The group prefers that the individual optimizes for the group's specific facet, and exerts significant pressure on the individual to do so. The group does this because it wants to be successful, not because it's on a power trip, and having the individual members optimized for the group's purpose helps significantly.
In the past, people have always optimized for the group. Spec, loot systems, loot priority, role, professions, buffs. Dual-spec systems will be no different. The system Blizzard is outlining will succeed with Goal 2, but fail to meet Goal 1. Both specs will be dedicated to a single facet of the game, especially for hybrids.
The only way to meet Goal 1 is for the game system to mechanically enforce the separation between facets.
For example, if one spec was only enabled in instances and the other spec was only enabled in Arena and Battlegrounds, then that would enforce an individual's ability to participate in all facets of the game. A group cannot pressure the individual into using two PvE specs, as the game physically prevents it.
The price here is that you lose the flexibility within a facet, you've now failed to meet Goal 2. Even so, I suspect that most individuals would probably prefer this solution, to be able to switch between PvP and PvE, or raiding and farming.
There are other solutions. For example, if you expanded to being able to switch between 3 or 5 specs, you could do:
Spec 1 - enabled in PvE
Spec 2 - enabled in PvE, world, PvP
Spec 3 - enabled in PvP
This would allow you to participate in multiple facets, but still have some flexibility in the group situation of your choice.
The ideal amount of flexibility would be something like:
Spec 1 - enabled in PvE, world
Spec 2 - enabled in PvE, world
Spec 3 - enabled in world
Spec 4 - enabled in world, PvP
Spec 5 - enabled in world, PvP
Note that spec 3 simply cannot be used in PvP or PvE. It's a guaranteed farming or personal spec. The game needs to enforce separation of facets, and prevent one facet from monopolizing all the character's resources. However, this is not as simple a solution, and might be too much flexibility.