My feeling is that in a community like this things work best when software add-on by community members are distributed freely. Part of this is that...the idea of original code is suspect - most people build on other examples and unless you are coding in a machine language - well, should the "creators" of javascript get a percentage of your income?
But beyond that argument, I think it is amazing the kinds of things that can come out of an open and free community, as we have already seen. With the addition of shaders we will get to enjoy another method of participation and sharing. I can't see charging money for these things as being a positive development. It also raises the stakes for the developer and, in a way, puts some responsibility on Scirra, if they choose to share in the profit.
The way it is now, developers can make early releases, let the community beta test it and find bugs, and iteratively develop it on their own schedule, and everyone wins.
Art assets on the other hand make more sense to sell. It is easier to screen this process as well. If the quality of the art is poor, don't support it. With code, it is harder for both the consumer and the host to determine whether it is high quality.
All just my opinion and I'm very receptive to the idea my opinion is a lousy one.