As I've been reading throught he first few "chapters" of Ready for Anything, I've had a few aha moments regarding Prioritizing which has shed light, for me, on the perceived friction between GTD and the Franklin Covey approach.
It occurs to me that the concept of "priority" is multi-factorial. (wow, I've been deposing too many scientists lately). The concept includes both an urgency factor, and an importance factor related to some goal or area of our lives. It's the combination of these two that causes difficulty.
The ABC priority system forces the user to meld the two factors together, without any clear formula for doing so. The Quadrant approach forces the user to explictly separate the two. That's why the merger between Franklin and Covey was so bumpy. But in either approach, you're tracking "priorities" in your system.
What I've come to realize is that when David talks about NOT tracking your priorities "out there" in your system, he's referring to the Importance factor. When GTD users object to this, they're usually thinking of the Urgency factor.
DA has no objection to tracking Urgency, in terms of the time-sensitivity of a project or action. (I think this point is alittle more clear in the CDs than in the book). Indeed, that Urgency factor is essentially static. The due date is what it is. To the extent Urgency changes, it is an external change caused by the inevitable march of time.
However, the Importance factor is purely internal and therefore extremely dynamic. That is the factor that cannot be systemitized.
So maybe we need to change our vocabulary and talk about "significance" rather than priority. This is just my thinking. Hope it helps.