1) My typo :P (the original list was a bit different and did a C&P error)
2) I raid lead (even if at a modes level) so I usually have no problems in getting raidspots
3) read the class descriptions in
http://www.worldofwarcraft.com/info/classes - there is a difference between "Paladins are a defensive class designed to outlast their opponents" and "Rogues are a lightly armored melee class capable of dealing massive damage to their enemies in a flurry of attacks" (just as an example)
Everyone can mostly be replaced by everyone within their roles, meanwhile raid leaders spend their time trying to find half decent players of any class, that can use their brains at a decent level.
To a point all classes and specs are different but there is a balance line - survivability, damage ability, utility and sustainability. That is why, even within the same class, different specs should have different DPS potential. If a class/spec equals another in everything but for one of this factors - where it is superior - you just found a superior line up - the other one is redundant. That is the reason that - for example - a fire mage should out DPS a frost one - the latter has more utility, survivability and sustainability. The ability to switch roles is - in itself - a personal utility. Which is quite relevant on a guild setting (e.g. - I have more spots in the guild for druids than warriors).
As classes go the skills are less comparable but they translate into different "utility" values. If there is one lineup for a particular role that is both stackable and superior, you get "stacked" raids effect - which translates into overrepresented classes.
Now to the point - should a retri pala have competitive DPS? Sure, no problems with that. Hell, I had no problems in having retri palas in my raids, as long as they did know how to play the role (which was the case). But what is "competitive DPS"? Not necessarily "equal" (I'll let you on a little secret - DPS has relative value as long as it is decent -I don't get particularly worried if someone deals 80% DPS of the top dmg dealer in a raid - extremely concerned if it is bellow the tank :P)
So, should a retri pala match the dmg potential of - let say - a rogue? No, nope, sorry.
There are several good reasons for that - some RPG connected (yes, I know that wow is getting infested by Action figure hypnotized CS kiddies but no need to overdo it), other due to the class descriptions (Blizzard does states that rogues are at the highest level of DPS values) - but also to avoid class stacking.
There is not "retri pala" class, but Paladin. All aspects (protection, holy and retribution) are a part of the deal.
And there is only room for 2.5 players per class. Which means that each raid should have - on average - 2 or 3 palas/mages/.../DKs. And raid/class design should reflect that. Not on a "need basis" but on a optimal basis.
But what is the value of role flexibility and how much should it cost in terms of DPS?
Well, it is hard to put a figure on that, particularly taking into account that different specs blurr the lines further (e.g. a "survival" spec of a "pure DPS" spec could be bellow of a DPS spec of a more hybrid class). But, regarding dmg output if one cannot see why classes that specialize on it should have it at the highest level, well...