Except there wasn't really alternate equally working set of abilities. There was pretty much one obviously best set for each trait tree.
There actually were in the early days. Before the simplification. I played a mage and there were a lot of different builds you could play, and none of them were "the best". There were 3 talent trees, arcane, fire, and ice. Couldn't play fire in some raids, because bosses were fire immune...but not all of them. Some people played an "elemental" build where they went half way down fire and ice trees and had amazing control and still some good firepower. Some went full ice and got major control in pvp. Some went a mix of arcane and fire...some more fire, some more arcane. Mages back in TBC had _so many_ good builds and strange tricks that it was amazing. But the developers said that it was "too hard to keep up with" and they couldn't "anticipate all the builds" and there were times where things got quirky and a strange build popped up that was absolutely killer. His statements aren't opposed to each other at all. There were complex systems in place, and no -- it wasn't balanced. It was a rock-paper-scissors type game at the time. Mage destroyed warrior. Rogue destroyed mage. Warrior destroyed Rogue. You relied on having friends around you to keep you safe.