My new Alliance guild has one other level 60 priest, who refuses to heal. Not only does he not want to be the main healer in a group, he doesn't heal *at all*, staying in shadow form all the time. Besides that, he is constantly complaining: Priest gear only gives good bonuses for healing, not for shadow damage. He isn't invited to groups as often as he would like. And so on. At this point half of you, my readers, probably think "what an idiot to make a shadow priest", while the other half thinks "right, why doesn't Blizzard support off-spec talent builds better?".
The issue is a complicated one. World of Warcraft not only has specialist and hybrid classes. But each class also has different talent builds: Some that make a specialist even more specialized in what he does best. And others which turn him a lot more hybrid. And attitudes toward non-standard off-spec talent builds vary a lot. Obviously as long as you solo, going towards hybrid might even be the better option. Many a holy priest has leveled up to 60 being shadow, and will switch back to shadow the day the Burning Crusade comes out, and he has to level to 70. Off-spec talent builds are also often better for PvP. But groups, and especially raids, nearly always prefer the standard talent build that turns a character into a highly specialized expert in his field.
Of course that also depends on how many of each class are in a guild or a raid. Especially warriors in raiding guilds often can get away easily with being specialized to deal damage instead of tanking, because a raid only needs a limited number of tanks, and warriors are plentiful. If a guild had far too many priests, some of them dealing shadow damage instead of healing wouldn't be much of a problem. But as priests are generally in short supply, they are all needed for healing duties. The one guy who refuses to heal often just isn't welcome.
Yesterday's paladin thread suggested that the same is true for retribution paladins. I only have a warrior and two priests at 60, so I really can't say about other classes. Is destruction spec considered acceptable for warlocks? It always appeared to me as emulating a bad mage. But then, in a raid warlocks are needed for very specialized tasks, like banishing, which aren't influenced a lot by talents. It appears that feral druids are about as popular as shadow priests, that is to say highly annoying to a raid group.
Blizzard supports those off-spec talent builds only half-heartedly. For example my priest is running around in mostly prophecy gear, the epic tier 1 set from Molten Core, like many raiding priests. And a prophecy set gives a lot of nice bonuses to healing, improves your Flash Heal casting time, the critical chance of your holy spell, etc., but doesn't really do much for shadow priests. A shadow priest would be better off in a set of warlock tier 1 armor, but of course he can't wear that. It appears that in AQ40 a lot of gear drops which is good for off-spec talent builds, but AQ40 is a lot harder than Molten Core, and not as frequented.
But the main problem of off-spec talent builds is a social one. The more strict hardcore raiding guilds won't even invite shadow priests and other people with non-standard talent builds. It is the classical conflict between individual freedom and the greater good of the group. Most guilds face a shortage of healers, and a shadow priest refusing to heal is a slap in the face of the other guild members. He could be useful by a simple respec, but refuses to do so, and insists on being a mediocre damage dealer instead of a great healer. That has obvious advantages for him, when he wants to solo or do PvP, but comes at the detriment of the guild, and thus causes friction.
Part of the problem is the decision of Blizzard to put a brake on switching between talent builds, by making it more and more expensive. Compare that to a game like Final Fantasy XI, where at any time you can go back to the city and switch from one class to another without cost. Off-spec talent builds would be a lot more acceptable if you would for example go shadow for PvP, switch to holy on raid night, and go back to shadow afterwards. By making that hardly possible Blizzard is trying to artificially increase the number of character classes. But that comes at a cost of creating character classes that are just not welcome in many guilds.