That is a tough decision to make.
To make a public board or just make it private.
So you need to make a business case on the subject and weigh the pros and con's.
First on the list is:
Do I want more traffic.
If yes then you need to open up permissions for guests.
To allow spiders and casual visitors to see your content.
Con:
By doing so you encourage spammers ,trolls and problem members.
Like I said before you can't have both.
You can take steps to mitigate some issues but not all.
Things such as a guest can only access certain forums.
Can't post till registered.
ETC.
There are some members here that want the opposite.
They want their board completely closed to visitors.
I for one don't care about visibility for one forum I take care of. Because it is dedicated to members of a organization only.
But I leave it somewhat open so guests can see something and participate as guests using captcha.
