I think a good way to solve this would be to make Default rules almost like ReadOnly rules - the only allowed action would be to enable or disable them. In addition, add a button to clone a rule, and you will always have the defaults on hand, while also being able to quickly use them as a basis for customization.
NOTE: Upon clicking this link 2 tabs may open - please feel free to close the one with a login page. If you only see 1 tab with the login page, please try clicking this link first: Authenticate me! then try the link above again. Ensure pop-up blocker is off.