Good post. I was on paper in favor with this concept of "hybrid tax" but your post made me realise there is a real dichotomy between:
A) Bring the player not the class.
B) Hybrid tax.
However, I think "Hybrid tax" is both a misnomer, bad politics, and even likely has no link to its implementation.
The easiest way to see this is to imagine everyone playing the exact same base class but bringing different buffs to a group situation. As WoW has evolved, the categories of buffs have been reallocated from one set of players to now as many players as possible. However, the only "buff category" which has not been reallocated and no intent to reallocate is Damage. Blizzard have decide to keep the Damage buff category to pures exclusively. In other words, when you bring a pure to your raid, you're not just getting the normal buffs but you gain an additional Damage buff. The cost of that Damage buff to the player and the group is the lack of role-changing flexibility (not total lack mind, since Blizzard decided not to include warriors and dks as hybrds). Therefore,
"Pures Damage Buff" rather than "Hybrid Tax" (a positive rather than negative intent) is better politically and likely more accurate name due to implementation.
There is an argument that if Blizzard balance around the Damage Buff category for pures against the environment, then for pures their damage is a type of survival buff since you can eliminate an "ordinary" threat before it kills you much like you can outlast the threat as a hybrid since you are able to heal yourself. However, I think the game has come a long way since inception and this form of balancing is minimal not least because there are now a full 21 viable dps specs in PvE.
That said, there is another category of buffs which has not been reallocated: crowd control. Since the importance of this category is returning in Cataclysm I really do wonder if Blizzard have taken into account that the values of pures in a non-nuke-or-gtfo-WotLK environment is a lot higher. Half the problem with parts of the previous expansion is that some classes even well before the endgame, namely mages and less so rogues, had enormous damage compared to other damage dealing specs and yet had the strongest battlefield control. This reduced the demand (drastically in some cases) for hybrids who played dps specs. The only hope is that Blizzard have come a long way in tightly controlling class balance and buff balance since then and the likelihood of one class being so far ahead that it fills most raid dps spots by itself is fairly small.
However, I will be levelling a mage in Cataclysm, just in case!
In respect to shamans, I cannot put it better than this its so blatantly obvious (since Blizzard have said it themselves and are trying to figure out how to fix it in Cata - especially for enhancement):