Do you charge your members for anything? Do you charge them for being a member on your forum(Some forums make members pay a monthly membership)? Do you have a premium membership program? Charging them to view special content that other users can't?
I have "gold members" that get a handful of perks like a larger avatar, a pretty coloured name, and a custom usertitle to name a few.
I think anyone should be allowed to join a forum free of charge. But it is always a nice addition to add a paid subscription for extra features regular members don't get.
I really think that the issue deals with what kind of target market you have. I mean, if you have a forum with a bunch of teenagers, how likely is it that they are going to pay for membership and what form of payment? I think it depends on aspects of the community.
Isn't that somewhat deceiving? I mean how would you feel if DP suddenly charged for membership? If I were a member I'd prefer knowing upfront. Totally agree in the end it will also depend a lot on your niche.
You can always ask for donations though, I do that sometimes and if you give good features, every now and then you can get a little.
If your site provides something worth paying for then it can be a good idea ... what is your site about anyways?