This may be a little off from the discussion you were looking for, but I'm going to contribute anyway.
I think a lot of people who get into GTD worry about priorities for a lot of reasons. First, if they've always used them, it's hard to let them go. It took me a long time to really get int GTD for that reason. Now, I won't go back.
The other reason that I'll mention is that GTD tends to come with long, scary lists. People don't want to lose things in the depths of the lists, so they gravitate toward strictly prioritizing everything.
Here's my compromise: I only use priorities to bump things that I want to see to the top of my list. I have no illusions about doing those things in that order. I usually don't. But it makes me comfortable with not "losing" things in my lists. I don't sit and prioritize every day. In fact, only rarely (in unusually stressful, I'm-really-really-in-a-crunch kind of times) sit down and assign lots of priorities at once. Usually, things change priorities if I see it at the bottom of my list and want to see it again really soon, or if I see it at the top and know I'm not going to do it anytime soon. That's all. Waiting for items and pending are all 5, because I often use the "All Categories" view to decide where I'm going to "go" next.
Really, in my experience, I haven't lost anything that way. I have a lot of to-dos -- enough that I don't like to scroll all the way down the list all the time. Yeah, my to-dos have different priorities, but I'm not tied to them. I just scroll the list until I find something I want to do, and loosely manage my list so I'm more likely to find something quickly.
I encourage anyone who's having priority issues to try GTD without using priorities in a strict, limiting manner. I've really saved a lot of time this way, and have been able to do more.