I think that this constant harping on how priorities change too quickly to be worth keeping track of is really falling into one of the traps GTD is designed to help us avoid: don't stop to immediately process a new task or piece of information, unless it's absolutely necessary.
My workplace is one constant interruption (as a technician, I am keeper of the lab's tribal wisdom!), and pre-GTD I was notorious for forgetting to do things I committed to doing. The one great thing it taught me that I didn't have to try to remember, and I didn't have to do it right away, but I could put it in an inbox and deal with it when I wasn't working on something.
If a priority changes, if I need to put one project on my back burner and work on something else, I don't run back to my computer right away to update my priorities. If I chip a tooth and have to call my dentist right this very instant, I don't go and upgrade my "call the dentist" task first.
If something needs immediate action, I do it. I don't fuss about whether my system says it's important - this is what the evening review is for. If something gets a priority change, but doesn't need immediate action, it goes to my inbox - a 2 minute task that probably will never see the inside of OF's database.
And in the evening when I review my day, if a lower priority task became immediate and got done, I just check it off, because hey it got done, and now it doesn't matter whether it was urgent or not.
It's useful information if you don't obsess about it. Not everything needs a start date or a due date, and not everything needs a priority, but if you don't recognize a task as important when it really is, it's useful information to have.
Originally Posted by al_f
That renders any sort of "high, medium, low" or whatever system redundant as far as I'm concerned: you'd spend longer readjusting your priority settings than you would actually doing the tasks (OK, I'm exaggerating there, but you see my point).