One of the things that's often surprised me while reading forum threads concerning dual specs is just how wide the gulf is between pure and hybrid classes' perspective on the issue.* Pure classes are worried that they are increasingly obsolete in an age where a hybrid can easily respec between tanking, healing, and DPS. With the damage differences between hybrids and pure DPS shaved fairly thin in Wrath, they've got a point, and it's not that unusual to see resentful players asking why their class -- which cannot choose a different role -- has to risk losing a raid slot to a class that can.
By contrast, most of the hybrids to whom I've spoken see dual specs -- not as a competitive advantage, but rather as relief from the financial drain of constant respecs. 50g may not seem like a lot of money in Wrath's economy, but it adds up, to the point where you realize that the two hours you spent doing dailies will only net you about 20g after, say, respeccing to heal a heroic and then respeccing back to tank or DPS for the night's raid. Thus hybrids are equally resentful about the demands made on their time and e-wallet that pure classes don't have to worry about.
The end result seems to be that players talk through each other. Hybrids are focused on saving gold. Pures are concerned that their class is just not desirable enough in the absence of truly superior damage. Hybrids chafe under the cost of building multiple PvE gear sets to do a raid or group's most stressful jobs. Pures twiddle their thumbs on the sidelines in the knowledge that, once a group or raid's DPS slots fill up, they will be sat in favor of a worse player who can respec to do a needed job.
In the end I think pure classes, oddly enough, have the most to gain from the dual-spec system. Dual specs will allow pure DPS to maintain PvE and PvP specs without having to pay to shift between two sides of the game. And while I think many PvP-oriented hybrids will do the same, there is a very strong financial incentive for hybrids to dual-spec for the most requested PvE roles (tanking and healing), which will increase the supply of players available and willing to do these jobs in groups and raids. This is pretty much a win-win scenario for a pure class. The effect on hybrids, while still very positive, is going to be tempered by the fact that PvP specs (or possibly soloing specs) would remain an additional expense on top of the usual need to build, gem, and enchant multiple PvE sets.
Personally I intend to run with PvE tanking and resto builds once dual specs go live as these are (by far) the most requested roles of my main, but I'm curious to see what attitude players are going to bring to the new system.
*For the purpose of this discussion I should note that pure classes = Hunters, Mages, Warlocks, and Rogues. All other classes can all spec into at least two different roles for PvE.