That dual spec should or shouldn’t be added isn’t contingent on how in-step with design philosophy it is, you keep making this strange assertion that dual spec somehow qualifies to be added simply because it “fits the design”.
I’m sure given some time and a round table, I could think of an entire list of features that would be “in line” with TBC-ish design, and I’d pitch that list for a Classic+ server no doubt.
This isn’t your mother’s potluck, though, where we get to just add any and everything that feels “TBC-ey”.
The whole idea is to succeed (or fail) at the game under the restrictions and guidelines placed on players during retail TBC, in an effort to mimic the experience.
Dual spec, whether it fits conceptually in to TBC design or not, adds a level of flexibility that was non-existent in retail-TBC. Adding it would decidedly allow for emergent styles of gameplay in terms of managing your characters build that simply never existed back then. Good or bad, it simply wasn’t available yet.
It will be soon though. WOTLKC is all but guaranteed.