Where this breaks down is in the case of DPS. Since there are no classes that can only tank or only heal (how would a 'pure healing' class ever solo anything? Heal themselves until the mobs got bored and dropped their loot in frustration? Go team cleric indeed) it's not seen as unbalancing if all the tank classes are at parity or as close as is possible with different abilities and different encounters. After all, even if there are four classes competing for your role you can always choose to respec to a different one, or at least that's the logic usually seen. But since the DPS classes only DPS, and can't respec to a different role, you have to give DPS classes a small edge as DPS so that they won't get swamped. After all, only four classes can tank out of ten, and only four classes can heal out of ten, but all ten can DPS. You need to give the pure DPS classes an edge if they're going to stay worthwhile.
The difference between hybrid DPS and pure DPS classes in Wrath was deliberately reduced. Whereas before, the pure DPS classes were expected to always be on top and be well ahead of other classes, now it's expected to be close so that a skilled, well geared player of a hybrid class should be able to put out competitive DPS especially when compared to a less skilled player of a DPS class. This does, however, lead to the idea that we're still seeing a hierarchy here: if you choose one of the four 'pure' DPS classes, and are as geared and skilled as a hybrid player, you should almost always have an edge on that player.
Since WoW abandoned the idea that there was one tanking class and two classes that could offtank or trash tank, and abandoned the idea that there was one main healing class and three 'also rans' that can heal if there's no cloth wearing healer available, it seems to me that it's time to abandon this last vestige of the old design. There's no reason that the choice you made at level 1 should help or hinder your DPS automatically any more than it should hamper your tanking ability or healing ability. If you choose to be a DPS player, you should be on as close a playing field as Blizzard can maintain while still keeping the classes flavor distinct and mechanics independent. But how to go about this?
Give all classes a secondary role at the minimum. Paladins and Druids will of course remain the ultimate hybrids, with the ability to fulfill all three roles in a raid, and perhaps Shamans can gain tanking viability too, but more importantly the four 'pure' classes will be given a role that fits their flavor and abilities. Since of the four DPS classes in question we have two pet classes, it's not hard to imagine Hunters and Warlocks becoming Tank/DPS with a revamp of the Beast Mastery and Demonology trees. But what about Mages and Rogues?
One possibility would be to break the trinity of DPS, tank and healing up in some way. We already see this to some extent as the physical DPS vs. magical DPS argument, or the ranged vs. melee one, but there's also the concept of mezzing/debuffing that other MMOs are more distinct about. WoW has this in the form of CC like polymorph and sap and debuffs like Curse of Elements or Blood Frenzy, but these tend to be stripped down in current play. In the current edition of the pen and paper Dungeons and Dragons game (which heavily borrows from MMOs) there are four roles, defenders (roughly analagous to WoW tanks), leaders (effectively the game's 'healing' classes), strikers (the damage dealers) and Controllers, who specialize in damaging or inconveniencing groups of mobs and fit into the mezzer/debuffer category.
Both the WoW Rogue and Mage could fill this role. You could spec to be a pure damage dealer with minmal crowd control and debuff options, or spec into talents that increase your ability to slow/root/ snare and weaken your enemies. Mages would by necessity be the stronger at area debuffing/mezzing while rogues would specialize in single target, which would probably mean they'd need stronger debuffs and more ways to apply them. In order for this to work, of course, we'd need to move away from AoE tanking as it currently stands: in effect, mages and rogues would assume some of the burden for controlling and debuffing groups of adds to keep them from swarming a group that is currently the role of the AoE tank using Thunder Clap, Swipe, Consecrate or Death and Decay. In a way, it would be a step back to the days when a five pull in Blackrock Spire meant three mobs would have to be CC'd. As such, I don't know if it would work in the current game, but the game has changed in the past to allow classes to gain new roles, it could do so again. Abilities like Tricks of the Trade point to one way we could allow rogues to help AoE tanking by giving their threat to other players.
In the end I wouldn't expect any such sweeping changes anytime soon. We already have a lot of classes in World of Warcraft who can do more than one role in a group, it's possible that's all we need.