Does he really have much to lose in trying the double? Is it possible for him to lose the 1500m by attempting both (I can't recall the schedule). If he feels the 1500m is close to a lock he doesn't have much to lose in trying.
Whould he lose votes in T&F NEWS Athlete of the Year if he goes undeafeated in the 1500m this year but gets 4th or 5th in the 5000m - if so that would be unfortunate. I really enjoy seeing athletes stretch themselves into other events. I would have liked to have seen MJ try an 800m, or Kipketer hop in a top flight 1500m, etc..
I hope he goes for it - in my mind if he wins the 1500m and gets dusted in the 5000m he loses no luster.