The gist of has been said is that if you are involved in an accident and being restrained by a rear seat belt, you really don't want that seat belt bolt loaded with additional impact from unsecured items behind the cage.
ps, if you are NICE to him P38A used to have a copy of the fitting instructions on PDF, my copy courtesy of Ron is at home otherwise I would post it up now for you to studyy and understand how it should be mounted.

