Ok few things here:
The site has to be hosted
their are people behind the clicks. I.e. the site was developed by someone.
The site needs to be updated
All of these i think are valid arguments as to why you may want payment. However, unless you have a group of dedicated peeps to respond to questions and you can guarantee answers, then I think the payment thing is out of the way.
This is a PHP thing not Micro-you-know-who = Opensoft mates (freedom of info)
Another thing to consider is paying everyone that responds to a query. Fulfill all of this then you can collect payments.
Oh... one final thing. Do your research well (this is a start I know) how many sites with forums
- collect payments
- Are as active; if you forum is inactive then it is usless.
Of course these are just MY opinion