Just watched this interesting video. Do I understand correctly that the airflow generated by the fan only cools the outside of the heat sink? It seems like that from this video (1:40). If so that takes away my worry of dirt flowing into the internals of my Mavic. What do you think?
Haven't looked at that up close and now my MP is on its way to Carson, CA

to have its camera replaced or fixed.
If you put your hand at the back (when the fan is running) you'll feel air coming out of the vents at the back. Any teardown video shows the fan venting into the inside which is the best place to pick up heat.
The video shows some flow from under that under-cap and that, if so, makes sense when the MP is in forward flight: the nose is down, so anything that helps direct airflow to the heat sink will help in cooling the unit. It would also help when sitting on the ground.
Yet another reason to use a good landing pad, esp. if flying off of pavement in the summer.