Not all that puzzling. G.W.B. took Delay on a high profile trip on Airforce One not too long ago. This is no different. Rough last few weeks, she withdraws and Card has taken some hits. Solution- Good PR face saving/still got your back trip on helo.
My guess is that Miers was very publicly invited onto Marine One and taken to Camp David as a face-saving gesture. I don't imagine she played any real role in the selection of her successor.
Of course I can't prove it, but surely her objectivity at this point would be questionable.
I'm sure the dear woman is acting in an administrative and technical capacity. Evidently, she is good at just that.
Sounds like the starting line for the neo-cons.