I think a pretty large one is that it simply doesn’t do a good job of solving the problem(s) it aims to solve.
I logged into Retail the other day, joined a dungeon group through LFD, was ported into the dungeon, and while out of combat with no costs (reagents or otherwise) was able to change my talents.
Dual spec is just like single spec, except it adds 1 spec. It still has all the same problems and benefits of single spec, but simply adds 1 more spec.
If the problem(s) being solved by dual spec are saving talent builds or keybinds, those problems are already solved with addons. If “I don’t want to use addons to do this,” then you don’t want dual spec, as evidenced in your lack of will to solve those problems.
If the problem(s) being solved by dual spec are “I don’t have gold,” then removing or reducing respec fees solve that problem (so does having more gold, by the way).
If the problem(s) being solved by dual spec are respec’ing in the middle of an instance, then Retail’s system solves that problem.