I guess the potential harm is that someone else could copy and then copyright your idea. As you continue selling your original idea, you could be accused of copyright infringement. Seems like a lot of work for someone to go through to put that on you, but I suppose it could happen.
I certainly wouldn't want anyone sketching the pattern at my booth, but like it's been said, with cellphone cameras, they can simply take a snapshot, download it, then copy it at home.
I won't plan to copyright unless I'm going to publish and sell my own patterns, which I would like to do someday. Selling patterns at my booth for quilts on display could be a good idea too perhaps?