I inquired with my insurance company about deminished value, they said that it only applies if you pay for the additional coverage. I found that state law doesn't allow for diminished value claims where I live. There are a few other states that don't allow for these claims too. It might have to do with the fact that Michigan is a No-Fault state.
Link with some info not all specific to Michigan.....
http://www.carinsurance.com/kb/content33391.aspx
How Diminished Value Works....
http://www.carinsurance.com/kb/content10766.aspx