In the forums I've administered, the bots never comprised more than, say, 10% of the total connections. I don't know what the ratio here is because I can't see that information. On phpBB, for example, the bots show up as named users.
That being said, I think restricting access to people who are just looking for an answer and not signing up is a bad idea. I know I would not have signed up at all if I had been required to. I think it's a big turn-off. I see a lot of people who signed up saying they did so because they liked the forums and how friendly the people were. I'm not confident they would have signed up if they were met with some "create an account to see stuff" screen.
And
that being said, it's not up to me in the least