For the thing about restricted areas being visible - yes - that would be true.
For security problems, I don't agree.
Spoofing a User Agent can work one way (client to server) [ and the reverse is true but not worth discussion here ], and spoofing IP addresses can only work one way (client to server OR attacker to attackee).
The solution around spoofers is to first check the UA, then check the IP. If the IP does not match an approved one, then deny access to the site. Otherwise, allow. That way, only real bots / spiders are allowed.
Spoofing IP addresses is only good for DOS/DDOS/ similar type attacks. It cannot be used to gain access.
I.E. IP address verification is sufficiently secure.
This is NOT something you would want to do for some things, but it would provide an incentive for people to sign up at forums. A similar tactic is common place at some current major sites.
They allow search engine visitors, but disallow internal searches without sign-ups.
The whole thing is quite clever I think.