I deleted one of my first Hubs. It had violated the rule of having duplicate content. It was my own piece of work though, published somewhere else, saved on my computer. I didn't realize it at first, took it down and then deleted it.
That way I didn't lose my piece of work for real and other Hubs I always try to improve. Deleting a Hub in general, is a drastic measure. I would do so if there's no other way of improving.