The reason that SF exists, and the reason that SF brings in money to the owners (Fok and J, presumably), is because of the members.
Without members who contribute useful content, SF dies and so does the income stream.
There's clearly a balancing act here - SF needs revenue to cover costs and to give Fok a wage for running the site and advertising provides that revenue stream, but if there's too much advertising it will deter new members and drive current members away.
As long as there is advertising on the site, I'd be vehemently opposed to the idea of a membership fee because, as a member who posts content (content that is, hopefully, of some small benefit to the wider forum), I and all of the other members who regularly post on SF are already contributing to SF's business model. Therefore, expecting active members to pay a membership smacks of double-dipping, in a sense.
It's a common model on forums. I'm personally opposed to the idea, as Ice stated before, but other members have suggested it, and I'm generally open to new ideas. But having talked to a number if people today, and hearing the feedback here, I'll shelve the idea. No media sure has successfully implemented a subscription/donation only model. Even Wikipedia can't do it, and they are looking at other revenue sources.
I'll tell you that my primary job In the organization is to advocate for initiatives that help the community - for example, @Synthese and I were hashing out improvements to b&s today, the major piece of development that we are getting resources for.
If you want things done, propose things that are realistic, things that I can bring to the table and reasonably defend.
Think about it. I am the community facing part of the forum management. I get to read and field all the complaints. If you think that I don't implement things just because, you think that I am more masochistic than I really am.