Doesnt sound like LUA, but man that would be sweet if they do ever make that change.
Love it or hate it, WoW UI modding brings a tremendous amount of flexibility and customization options.
I would guess the biggest hurdle is the number of problems this could expose. The UI to client interface might not be as clean as needed to give players direct access. And protecting against mods that "play the game for you" also takes time and energy.
If not built properly, LUA enabled mods would quickly be deciding which ability to cast next for maximum DPS, or who to target and heal next, with which heal, etc. WoW struggled with this for a while but eventually overcame it. I know, i built a few mods that exactly these things
All that said, I would LOVE to see LUA powered mods for EQ2 in the future. I believe its one of the greatest features of WoW, one that truly makes the experience a notch above any other MMOs.