i have to admit, i stole this from another blog about WOW, but its so damn good.
A question that's been haunting my mind. All of the full fledged, one role classes are able to do only 1 thing; DPS, while the rest of the player base are able to either tank, heal, or do all three roles. As I paid attention to the damage meters during my raids I never really saw a Hybrid do any great amount of DPS higher than a full fledged DPSer such as a Hunter, or a Mage, and vice versa. A Ret Paladin or a Fury Warrior are able to push out just as much damage as a Warlock or Rogue, but they're able to switch to a different role as they please.
Being a Tank, DPSer, and soon to be Healer, I didn't benefit nearly as much on my Hunter as I did on my Paladin, and he isn't even 80 yet. I didn't even bother dual speccing on my Hunter because either way I'll be doing the same thing. There seems to be no point other than switching between PvP and PvE specs, and I don't PvP. I could switch between two PvE specs but one will push out more damage than the other and it just doesn't make sense to switch to the lesser of the two.
Dual specs aren't the only thing that make Hybrids superior, as I stated above I don't really see a damage difference between the 2 class types. This means that a Druid with 2 sets of epic gear that's dual specced for DPS and Healing could likely be more useful than a Mage whom can either DPS or.. DPS.
Some complain that there seems to be nothing that makes up for the fact that one role classes have no advantage in that certain role over Hybrids, which can excel in everything. Every class has it's strength and weaknesses. For instance, you aren't going to get the CC with a Shadow Priest as you are with a Mage, and with Ulduar you can't just faceroll AoE everything anymore. Though you can argue that Hybrid DPS trees have their advantages as well, replenishment in the Retribution tree for instance. In the end I think everything 'somewhat' balances out.
I believe that right now Hybrids have an advantage over one role classes, and I'm sure it's challenging to balance this all out. It's more than just "Make this class hit more". It's complex mathematics and a lot of hard work. We've seen Blizzard Reps state that there won't be many more major class changes in wrath and I'm happy for that. I don't like having to replace all my talents every time a patch that changes them comes along.
Ending note: Make Ret Paladins take more than a quarter of the brain to play Blizz :)