I'll go out on a limb and declare the Blood Elf Paladin/Draenei Shaman a practical joke by the Blizzard team.
If both factions are the same, you cut down replayability of this game. If you decrease replayability, you reduce the length of time a player spends subscribed to WoW. If people are subscribed for a shorter time, Blizzard makes less money.
Right now, if a player tires of playing Alliance, they can roll Horde. Because the Horde have Shamans, the experience of the game, both in PvE and PvP, is different for each faction. If both factions have all classes, there is much less incentive to roll a new character, and more incentive to simply quit.
The majority of players do not raid, and thus are unaffected by faction imbalance. Solving faction imbalance in a way which reduces replayability for the majority of subscribers will negatively affect Blizzard's bottom line.
MMO's require subscriber retention. It is their entire reason for being. I believe there is no way any sane cost/benefit analysis would show that improving high-end raiding balance is worth the cost of decreased length of subscription time for a majority of the players.
That's just the financial reasons. I'm not even going to get into the damage this move would do to Blizzard's reputation as the masters of faction balance (see StarCraft).
Or the shattering of the balance of the 1-59 game, which most people believe is superior to the raiding game. Seriously, it's already hard enough finding a primary healer for a lower-level instance. How much harder will it be when only 2 of 9 classes are primary healers?
High-end raiding balance is simply not worth this price.