There are several things that I'm seeing here with the issue, as I see both sides of the story. I can see the positive in that a modder is compensated for his or her blood and sweat, and countless hours spent away from friends and family to pursue a hobby. In a sense Gmod is attempting to mimic Apple's business move in the sense of running an App store, something that is the current fad in computing. In a sense it shows the inability on Gmod's part to be innovative in a business sense; the site is copying someone else's business plan and failing to do their own thing. Will it fly? I don't think so. Even if it does get off the ground, I don't see it going far for several reasons.
The biggest reason is this: When I mod, my alternate personality and I don't give two flying flips about the money. I mod because it is what I enjoy doing. I mod everything, I mod computers, games, software, my car, nerf guns, etc. I take things apart, and I see how they work. I can't help it, it's what I do. I am an engineer both by birth and by training: I build things, I modify things, I fix things, and I bring things back from the dead. Heck, I have an awesome job in IT in where I get paid to do some of these things. But would I commercialize the modding communities, absolutely not because I know the people who are behind these communities don't do it for the money, they do it because it's what they love doing. Pure love of a job will always produce a superior product. It is the reason why I hate mass production, as things that are mass produced don't have the touch of a craftsman who loves what he or she does.
Commercializing the modding community would only break this spirit of free modding.
Another reason is the people that commercialization would attract. Already there are a number of people around, especially at the Neuxs, who have no appreciation for the work that modders do. Paid-for mods will invite those type of people who are never satisfied with what they buy or what service they receive, and will demand support and/or their money back if they don't like what they have. I had to deal with those people when I worked with Circuit City just before it went out of business, and those were the type of people that would make folks hate their jobs. And they are also the type that destroy free modding communities.
Basically, the long and the short of it is that I don't see a paid-for modding community working simply because of the people, and it's the people who in the end make the game successful.