AFK bandwidth hoggers keeping queues longer, time wasted on development workarounds, time wasted on support ticket distractions, speed hacks, and generally more misery for players.
Yes, there are upsides, too. But do you want to live with the above? Does Zenimax? I hope not.
This game is philosophically minimalist, not oriented to min/maxing. No floating damage text, no gear scores, no leveling treadmill in preparation for raiding. DPS meters, raiding aids, all of those relatively benign things that well-intentioned add-on supporters want will be dwarfed by the inevitable flood of the all of above problems if you add access to the code via add-ons.
Not worth it.