Blocking user agents does not provide security, but it does increase the cost of attack which is perfectly acceptable as a part of a larger defence strategy. I

Blocking user agents does not provide security, but it does increase the cost of attack which is perfectly acceptable as a part of a larger defence strategy. It's like moving SSH to another port -- it won't stop anyone who knows what they're doing, but the majority of the bots that blindly connect to port 22 on every single host that has it open will be stopped by it.