I don't know if discussing this here will help at all but I do hope at least some stock artists see this.
Ever since I started doing book covers for authors, I have had to consider one more thing to look for in the stock rules, available for commercial use or not. Some stock providers say it is OK, some say you have to ask them first and others don't say anything. In cases of background stock, there is always an option to look for other stock images. When it comes to models, the characters on the covers have to be very precise so finding the right model is very difficult. So once I find the right one, I try to work it with the model as much as I can. In my one year of doing this, I have had several instances where the stock artists never got back to me or did that very late (a month or more). Now you don't expect the client to wait for that long for you so these replies are really no good after so long. I have also had an instance with one of the biggest and best stock providers on dA where I wrote to her at least 10 times both on her dA and facebook and got a couple of replies saying I'll get back to you but she never did.
If you don't allow commercial usage, you can always respond with a no so the artist can start looking elsewhere and not hang around hoping you'll say yes 'soon'. And I'm not even talking about people not being active on dA, I am talking about those who show some or a lot of activity on their accounts. This kind of behavior leads to people not taking their rules seriously or in my case avoiding their stock even for personal work. I really hope dA includes some feature where stock images have a straight no or a straight yes mentioning whatever the stock providers conditions are for commercial use.
If stock providers are reading this, it will be a great help to the entire community of photo-manipulation if they keep a few things in mind:
1. Mention whether you do or don't allow commercial use of your work.
2. If it is 'ask before using' then respond as soon as you can.
3. Mention your terms instead of asking the artist. If you want payment, say it. I like how straightforward stock providers like jasonaaronbaca are about the terms. No confusing the artist, straight $20 and other conditions.
4. Check the artist's gallery before responding if you are skeptical about how your image is going to be used. The quality of their work will give you an idea of what your image is getting into.
5. Mention the fastest way to get in touch with you in your rules. If you check your email more often than your dA notes, mention that.
6. Thanks to Dani-Owergoor for mentioning this. The same thing applies to use outside dA as well. Either mention it is fine to display artwork on FB, personal websites with so and so places or if it is 'ask before uploading' then respond in a timely manner.
I really hope this helps both stock providers and other artists to grow in their fields and also to be a loving community.