There was serious concern with some evidence a few years back that the Dropbox client was touching files in folders that weren't shared. Not sure if that is what OP's concern is. But I agree with your point, the client is only one small piece of the puzzle and won't reveal much.
This concern pops up every once in a while because people like to put it out of context. Most operating systems don't allow any way to poll for file events in a specific folder only, so you either keep looping through all files in the folder looking for changes which would be an annoying resource hog or you get all events and ignore changes that don't occur in the folder of interest. It's completely fair to say that any installed application can do whatever it wants so you are concerned, but the touching files part is just people wanting to complain.
