"There is no need..." is a cop-out and a non-argument. There was no need for dual-spec. There still isn't. There's a clear double standard in works...
Both changes add nothing but convenience.
Dual-spec was introduced because it added convenience.
Tri-spec will not be introduced because it adds convenience.
It doesn't make sense.
Uniqueness is also a non-argument. Talent trees are shared between specs and you are already capable of being 2/3 of all your potential specs at once. Every class (except Druids) are, literally, 5 minutes from being completely identical in spec choices.
What do those 5-10 minutes of respec'ing, reorganizing action-bars, etc... add to the game? Inconvenience.
What does tri-spec offer? Convenience.
There's no solid reason NOT to add it. It removes nothing you'd want and it adds what you need. Convenience. What's the problem?
If you're against tri-spec you better prepare a damn good case supporting the introduction of dual-spec, if you don't want to be labeled a hypocrite. You can't make one out to be OK while claim the other one would break the game. They're both the same.