Hello,
I think it would be a great idea to be able to see the activity if a person opens a subfolder that you are sharing. It would allow companies to better understand how their dropbox is being used and would help them identify trends to better utilize their dropbox.
Example:
We have our ToolBox talks housed in a subfolder in dropbox. I would be nice to see when or the number people were accessing this folder. This would allows us to address how we are sharing items to make sure the correct messages are being received.
Craig