I disagree. If you have board and they are interested in the material they aren't going to leave just because they have to do a simple thing like sign-up to view the board before the first 20 topics. Wouldn't your theory also apply to people that arrive at a board and the very first thing they have to do is sign-up before they can view anything on it?
I am agree with you mate, I am blogging professionally and nowadays I do not use SMF as community for lurkers, but as content sharing platform and extension of my Blog.
I am the king, I am writing
the valuable content , search engines love me, I have set five threads max limit for guests.
Google and many more search engines they are all day long in my Blog and forum.
You are getting too much misinformation from others, possibly they do it on purpose because some one else prior them, had the good idea and made the mod and NOW HE worth the credit.
SEARCH ENGINES
DOES NOT USE COOKIES ..... THEY CAN NOT BE LIMITED BY SUCH A MOD.

I am an electrician, if you need help ask me instead