keeping records at your house is for the paranoid. unless you are the custodian of records, they arent coming to your house looking for paperwork.
content providers that include id pics and model releases are just catering to webmasters (understandibly) need to feel safe.
the law is basically subject to interpretation, as is any law ... and the only time you have to interpret it is if johnny law knocks on your door (highly unlikely unless you are doing something blatantly illegal and on top of that drawing massive amounts of attention to yourself). And if you did not produce any of the content, you show them the license and point out that you are a webmaster and the custodian of records for the content in question is:
joe blow
123 whatever st.
as is listed on your license you received from provider.
that is my interpretation of the 2257 law - ive read it 100 times at least ... but im not a lawyer ... only reason i can see wanting your own copies is if you are widely known (publicly traded companies always wanted ID's, releases) and wish to be your own custodian of records ... or you dont trust that the content provider is going to be around ... but responsible content providers will maintain records and inform clients of any changes (if address changes) so that records on your site can be updated.
wow this is getting long, and remember im not a lawyer
Mikey