In an event with lots of falling-on-your-face potential - and onto rocks - I would not want to be holding anything in my hands that might slow down/inhibit the normal reaction to put my hands out. Whilst a thumb compass might not inhibit this normal reaction I doubt it would last long; I would not carry a map in my other hand unless the route was complex and the terrain forgiving.

I wore gloves in the Great Lakes Race - these were fantastic as there was lots of handling of rough rocks; I did not have a fall though there were several close calls - wearing gloves means the natural instinct to handle rocks and to put your hands out if you trip is not inhibited.