I wish it could be used to lock in exposure compensation. This way I could shot a soccer match or field sport that is in sunlight on one part of the field and shadows in another part. This shooting always is the most challenging for me.
I recently shot a game with the two teams and the uniforms were all unbelievable bright neon sort of yellow for one side and all black for the other team. To make it worse he game was in and out of shadows, cloud cover and bright sunlight. Hello manual exposure and hit or miss highlights.
You know what would be cool? To use the button an exposure memory. It would be similar to the older MF lens where you could prefocus and 'memory' it thus allowing you to quickly change to that focus distance when required.
However, instead of focus, if they made it for EXPOSURE it would be great for shooting in high contrast scenes with a mix of bright light and shadows.