Have you ever been to the archives and done any research there? I have; it's not the most secure place in the world. I spent the day there several years ago digging up Civil War records. You determined what you wanted and submit a request. The documents were brought out and you and a bunch of other people went into a somewhat crowded room with desks and sat down to look through the package. Mine contained records for great-great grandfathers and one set from Revolutionary War - like the signed documents for bounty land. There were monitors, but it would have been easy enough to stash a couple of things away or deface documents.
What's worse; there had been flooding weeks or months previously, and some documents were lost entirely or damaged - nothing I was looking into, but power to the workstations still hadn't been restored. The best I could do was summarize information on a laptop until the battery ran out. Unfortunately this was before smartphones with cameras, so I couldn't even take pictures - seems like other cameras with flashes were not allowed, and the copier was overwhelmed and way overpriced. As far as security, you go to the desk, show some ID, and they snap a picture and provide you an access badge to wear - you keep the badge ... in case you might want to make your own.
Overall I think NARA probably does the best with what they have, but I don't get the impression they are fully up to the task. No doubt presidential records are treated differently, but who is to say that it would be impossible for someone to forge credentials - or worse to have valid credentials that were never voided after an administration ended. DC has a problem with forgetting about the "need to know" and shutting down security clearances when a job ends. The Archives may be able to limit the theft and potentially the sale of documents and memorabilia, but it's no place for the repository of classified documents - you are talking a home for historians - not a place for securing classified files.