To charge a member fee, you need to have certain parts of the site members only, or get some sort of discount scheme going with suppliers. Obviously arranging discounts with suppliers bring a f**k load more work for the powers that be and can turn into a semi full time job.
If it's about generating revenue for BC, then suppliers sponsoring sub forums can work well.
To make the whole site fee paid member only would kill it virtually overnight.
all comes down to whether the owners want to run it as a hobby or as a business really, but with the other thread going that mentions guitarchat.co.uk / drumchat.co.uk I think that answers that one!