Although I agree to a part with your post that there will always be a disparity between specs and classes, there always will be, I'd make the distinction that if you are trying to progress at a decent rate in your raid group as a dps warrior, I don't see why you would gimp yourself and your raid group by doing 6k (to re-use the numbers you used) less than what you possibly could be doing, simply by switching to a more optimal spec.
Saying that fury/arms aren't viable might be a stretch but they are certainly less desirable if you can do substantially more dps with a different spec within the same class and I would be very surprised if people in some guilds (depending on progress level) aren't asked to go gladiator on fights simply because of that reason, it's only logical.