I believe the point is that if seeing the file you know it's an unsecure yet sensitive file if it's plaintext and will therefore treat it right. Were it obscured the regular user wouldn't realize the file had to be treated with care since he or she wouldn't know it wasn't secure and wouldn't know what it contained.

Now the reverse would also be true, a criminal might not know what it contained right away if it were obscured, but it's a safe bet he would steal the obscured files anyway and go play.

This doesn't mean there's no point having any kind of security system but the very best though. Awareness of how secure you truly are is what's important, what/who you are protected against. Obscurity/weak security really doesn't help this in most cases.