"You never DPS. You're always tanking. So what if fury and arms aren't as good?"
I had a hard time answering this until I logged onto my old Horde server and talked to Elle, a friend of mine who plays a warrior. Elle has never played prot spec seriously, doesn't like tanking. A discussion of the state of the various trees made her unhappy, and she made the point that even if prot were capable of good DPS, it wasn't a playstyle she wanted for her warrior. This makes sense to me: she's always leveled as fury or arms, generally, and she's done so in a guild with at least two serious, dedicated tanks around most of the time. The thing about tanking is, you only need so many. In a guild that fields at best a 10 man raid by itself, two or three tanks is plenty.
With paladins, druids and DK's as well as warriors, that's four tanking classes, all of whom are expected to be viable for trash, offtanking and MT positions. Warriors are one of the most played classes in the game, so it's clear that with four tanking classes not all members of these classes are going to be tanking. And two of those classes can only DPS or tank, there's no healing DK or ranged caster DPS warriors.