In reply to pooriast.

If you keep the unused themes updated, you'll be ok. The problem is that most people don't update themes they're not using. If one of those updates contained a security fix, it won't be applied to the theme. You should read the historic TimThumb attack and how years later, people were still being affected by it because they forgot about the themes that had on their site that contained the TimThumb script. This post by Joost illustrates the importance of updating everything.