What do you want to see when you have "doing time"?
I think the bottom line is what do you want to see on your list of choices when you have doing time. This is my understanding of the intent of contexts, time, energy level and priority. If priorities are a bigger factor than where you are or what resources are available to you, then priority coding is the most effective way to narrow your choices when you have doing time. Which, of course, is different for everyone and the beauty of GTD.