I'm happy to pay for a membership, even though I haven't used the For Sale forum yet, as I feel it's my way of giving back. I can't put a price tag on the amount I've learned from this site... but I wouldn't have those same feelings if there weren't people I could learn from sharing what they know.
However, I can understand someone like them, who are almost entirely contributing to the rest of our benefit, not wanting to pay for a membership. I mean heck, I don't think any of my college professors would have been keen on the idea of paying to teach me (otherwise I'd have saved myself about $100k

).
That is how Spark, albeit indirectly, benefits from having people like Kevin or Brian around... by having people like me or the guys with blue names posting their first knives who, with proper guidance (from people who contribute here), may one day soon spend $75 on a membership.
Now, I do believe the rules against "advertising" have some merit. Nothing is more annoying than forums full of posts that are thinly veiled advertisements or signatures containing more information than the average post. The problem is... where do you draw the line? Who gets to decide who gets a "free pass" to post links to their websites without a Knifemaker subscription, and who doesn't?
The problem is... the definition of "advertisement" is about as grey an area as it gets. Who decides what is and what isn't? Technically, posting any completed work, showing a WIP, or even a username can be... and often is... construed as advertising. If you really don't want people to advertise without paying, it may as well be a pay-only forum... but I don't think this place would be around too long if that were the case.
Not that I'm necessarily for adding additional confusions by having more tiers any more than necessary, but perhaps some sort of "Valued (subforum) Contributor" tier would work. Something without direct selling/advertising privileges, but with a little more loose guidelines on posting pictures/websites while in the particular subforum. Set an appropriate cap (or give out one or two a year), take nominations, vote on who receives them.
I suppose we could always pool up and purchase Knifemaker memberships for those people, however that might just make people feel like a charity cases... and probably won't make them any more likely contribute.