I understand DA's point about why pre-prioritized daily and weekly ABC/123 prioritized Todo lists don't work. (indeed I struggled with bumping things from day to day in my FranklinCovey Planner days).
However, it seems to me that, assuming Collection, Processing, Organizing and Review have all been appropriately completed, the priority of a given Next Action on my list relative to each other Next Action on that list (and all other NA lists) is pretty well fixed (at least in the short term). If something on my NA list assumes a time-sensitivity, then it would go off my list completely and onto my calendar.
For me, what gets in the way of working off of my lists is the inevitable and frequent "interruptions" by my boss with new tasks/ideas/etc. At that point I need to weigh the priority of that new input against all my existing commitments, but that doesn't change the relative priority of my existing commitments vis a vis each other, it merely factors the new one into the mix.
So I'm not sure I understand the reluctance in GTD to prioritizing the tasks on the NA lists. Such prioritization would still not prevent one from making a conscious decision to spend time on a low priority item (e.g. because you're fried).