Druids have 4th spec because they had a DPS and a tanking spec in a single tree. A good argument, but what about Shaman and Warlocks? Shaman tanking threads pop up numerous times on the Shaman forums here on MMOC, and they pop up numerous times on the official forums. Warlocks have almost an entirely 4th spec with Dark Apotheosis, which makes adding another spec for them seemingly a no-brainer.
At one time, Shaman were a tank on the same level as Protection Paladins and Bear Druids in vanilla. In fact, Shaman were originally the opposite of Paladins. Both classes were supposed to be mirrors of each other, with Shaman being horde-only and Paladins being alliance-only. However, Blizzard eventually reduced Shaman's capability to tank, and reduced Shaman tanking ability over the course of subsequent expansions. Along with this reduction came the slow loss of the Shaman tanking abilities and talents. I definitely feel that Blizzard's decision to grant Paladins and Shaman to both factions, and make Shaman and Paladin completely different classes was a good idea. However, dismantling Shaman tanking was a pretty bad idea, and has led to some pretty wonky class design ever since.
Shaman are masters of the four elements (earth, fire, wind, water), yet only 3 elements are fully represented. Elemental is fire, Enhancement is wind, and Restoration is water. This is backed by their dominant abilities, Ascendant forms, and weapon imbues. There is no Earth spec because Earth represents defense and protection. Its a pretty silly omission, but it makes sense given the 3 spec limit. 4 specs would flesh out all the elements, and make Shaman a more complete class.
Finally, there's quite a few abilities in the Shaman toolkit that just seem out of place. The biggest example of this is Rockbiter weapon. This is the only weapon imbue that has no clear purpose. Flametongue is the Elemental primary imbue (and the secondary imbue for Enhance). Windfury is the primary Enhance imbue. Earth Living is the primary imbue for Restoration. Frostbrand is the PvP imbue. Rockbiter? You supposedly use it in the rare case of saving the healer from death or something... Rockbiter even has a taunt when it is unleashed, and the talent Unleashed Fury reduces damage by a further 40%. A great utility that most Shaman never use.
The case for a Warlock 4th spec is even easier to make. Again DA is pretty much a separate Warlock spec wrapped up entirely in a glyph. DA not only gives the Warlock unique stats, it also gives them unique abilities as well. Abilities that can only be activated by using the glyph. Blizzard however decided that glyphs shouldn't determine a tank spec. A good decision, but then why not just give locks a 4th spec so that Warlocks could tank, and the traditional DPS specs could be preserved? It's a win-win for everyone involved. It certainly helps that DA is reminiscent of Dreadlords from WC3, which is cool in its own right. I would hate to see DA's uniqueness get destroyed in the next expansion because Blizzard can't do anything with it because of the 3 spec system.
If we can't have all classes getting 4th spec due to balance reasons, we should at least consider 4th specs for classes that need them the most. That would be Shaman and Warlocks.