A pretty substantial point of debate when it comes to Hybrids is their place in PvP. The thought that only one spec is viable per class is most problematic in the context of Hybrids for a couple of different reasons. Some argue against every spec being arena viable, some argue in favor of it.
The argument in favor of each spec being viable that I feel is the strongest is the fact that each Hybrid spec is vastly different from all of the others. In the arena, an Enhancement Shaman isn't doing the same thing as an Elemental Shaman. They are doing similar things like dropping totems and casting Shocks, but the role they play is different. Enhancement is in your face, Elemental throws lightning at your face, and Restoration is healing your face. This is not like Rogues in which all three specs are about hitting you until you fall down, just in different ways.
Why is it a problem that all three of them aren't viable? It essentially feels like a forced block on your chances at success. "Just spec Resto" isn't really a valid answer, because it means you are not able to play your preferred role. Someone who rolled a Shaman to Stormstrike people's faces off are told to go get some Elemental gear, and that isn't very fun. To be successful in the arena, they are more or less forced out of how they want to play the game.
This is not necessarily a product of poor development. As was stated at the WWI, generally the devs don't choose which tree is the PvP tree, and which is the PvE tree. At least not when development starts. One evolves that way as players progress through the game. It's an MMO, so players will always try to min/max and find the best possible talent choices, specs and combinations. To keep the Shaman example going, Enhancement is alright in PvP in certain compositions. Elemental is just better. Thus, Elemental becomes the PvP spec for Shaman. There are definitely compositions that make an Enhancement Shaman very powerful, but they tend to simply be not as versatile as other Shaman specs.
So while the class isn't poorly developed, the nature of multiplayer gameplay brings out the inner min/maxer in us all. Elemental is just plain better in PvP. All classes have one or two specs that are the best in PvP, and one or two that just aren't very good there. This poses a problem on a personal level because, again, it means you cannot necessarily succeed while playing what you like to play. Of course, that also applies to PvE now and then. Not as much as it used to, but it does on occasion.
It also applies to real life. You may love to play golf and golf clubs hit golf balls pretty well, but you wouldn't use a golf club to play baseball. Buff golf? No.
Regardless of the arguments behind it in either direction, there's one thing that's clear: Not being able to do what you enjoy sucks. No matter if it's PvE or PvP, it sucks. The class and spec balance in PvE has improved vastly over the years, especially for Hybrids. Just about every spec has become viable. Even the weakest of them can still find a place in a raid, and that raid will still be successful. It's been made pretty clear that an active effort is being made to improve weak specs in PvP and arenas as well, though we have only seen bits and pieces of it in BC patches and Wrath previews. Season 4 won't be as balanced for the Hybrids as we'd like, but maybe S5 (or whatever Wrath's first season is) will be fine and dandy. We'll just have to see!
Again, it's worth noting that most players (including myself) are min/maxing powergamers. There will always be one spec that is preferred over the others. Always.