There's an RSA encryption cabinet in our datacenter at work, it has red rope around it like a nightclub line and a sign that says no non RSA certified personnel.
Almost as effective as the ssh banner that says "Unauthorized use is prohibited."
Security through hoping people won't step over a rope is no security at all.
It's my understanding that such login banners are used to provide support for prosecution efforts, should someone break in and claim they did not know that access by random people was prohibited.
I would hope nobody thinks such warning messages are an adequate substitute for human-based and technology-based security measures, any more than wearing garlic cloves or putting up "no guns allowed" signs are replacements for being wary of vampires and armed criminals.
I've only been in that datacenter once and I was signed in by someone with authority to help them with something. Like others have said here, it's more to help with "I didn't know I wasn't supposed to touch it" type cases. The RSA cabinet is the security, the rope is legalease.
Almost as effective as the ssh banner that says "Unauthorized use is prohibited."
Security through hoping people won't step over a rope is no security at all.