First, let's define the word "viable." In the context I will be using the term, viable means having a reasonable chance of succeeding. In WoW, viability refers to a class or spec's potential to properly carry out some task. This task could be simple damage-dealing to a boss, or defeating a PvP opponent. To be PvP-viable, a spec needs to be able to successfully survive as well as control and damage opponents.
It isn't enough to simply be able to do something - it needs to be done successfully. For example, it isn't enough that Entangling Roots is on my action bar - if I cannot successfully use it (because of game mechanics), then it is a PvP-nonviable spell.
Revisiting what I said
Yesterday, I proposed that any spec should be viable if it is its class's only spec that fulfills its particular playing style. So, if a Shaman's Enhancement tree is the only tree that allows melee damage, it should be viable for the sake of those who want to be melee Shamans. Asking them to respec would not be fair, because Elemental and Restoration offer completely different playstyles.
But is that enough? What if a Mage really loves Fire, and does not want to spec Frost or Arcane for PvP? After all, even if the role of the spec is the same (dealing spell damage), the playing style isn't.
I think it is safe to say a lot of players play for their spec and not necessarily just for their class. Should Blizzard make every spec in the game both PvP and PvE viable? That would be very difficult to achieve. Today, the game is already faced with many balance issues, despite that Blizzard isn't even trying to make every spec viable. They are more concerned with class viability than with individual spec viability.
The state of things
To address the issues of balance and viability, Blizzard has, for the most part, ensured that specs are viable for at least one type of gameplay (PvP or PvE).
And this is where I begin talking about us Moonkins.
There are several specs out there who give up viability of one type in exchange for viability of another. Subtlety Rogues, for example, are almost never found in a raid setting, but are extremely PvP-viable. Elemental Shamans are very desired in 3v3 arena, but would not be wanted in raids if not for their totems and Heroism/Bloodlust. Fire Mages have some of the best PvE DPS in the game, but perform poorly in PvP.
I can only think of two specs that aren't viable for either PvP or PvE: Frost Death Knights and Balance Druids. Blood is the DK tanking spec, and Unholy easily surpasses Frost in both PvP and PvE. Balance Druids have terrible weaknesses in PvP, and suffer from a broken Eclipse mechanic in PvE.
Moonkin: effort =/= reward
I would think that in exchange for poor PvP-viability, Blizzard would have made my spec great for PvE (like Fire Mage). So is it normal that my DPS only becomes comparable in a purely standstill fight, even though I exert myself much more than the average raider to maximize my performance?
Essentially, what I'm trying to say is that it is not fair for Moonkins to have to go through so much trouble to do the same as anyone else. Mages, Warlocks, Shadow Priests, and perhaps even Elemental Shamans, are all more PvP viable, and are all capable of more raid DPS with a lot less effort (save for Demonology Warlocks, except that they are more adequately rewarded for their efforts).
Reward should be directly proportionate to effort, skill, and knowledge. On my Shadow Priest, I can be twice as successful as my Moonkin in PvP, with half the effort, skill, and knowledge. It's really saddening.
I spec Moonkin for the sake of it. I guess I find it fun to go around as a giant chicken causing eclipses and typhoons. I also get a twisted pleasure out of going against the grain. In fact, I don't like seeing other Moonkins, as they make me feel less unique. But if I were to play for the sake of viability - I would just go Restoration. There is really no reason to spec Moonkin, unless your guild needs one for the raid buffs.
In Cataclysm, Balance Druids find themselves a little better off in PvE. This is great - but Blizzard: why did it take 6 years?