I would prefer a system able to give you reduction in art supply shop (reduction you could buy with points for example), with an overall coverage. DA delivery works probably fine in the US, but to get something in Europe it take between 4 and 10 weeks. If I need art supply I want it in less than a week.
If they somehow teamed up with an art supplies company, maybe. But I don't see dA doing any dA-branded art supplies, unless it was in limited edition like they seem to always do with other merchandise. And even then, it would probably be just a bit too expensive for what it's worth.
I think it's a cool idea, but I find DeviantArts shipping to take a very long time although I suppose you could compensate for that by ordering ahead of time?
It could be a cool alternative if you can't get them in more traditional manners.