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.