To expand on that point: some of us are not just "filling a gallery."

Some of us are creating specific threads of work which need to be categorized within our own bodies of art as stand-apart projects.
If this were offered as a feature, the option to create these sub-folders would create the opportunity for artists to present these stand-apart projects as individual and separate, self contained, works.

I see key-word searches as an insufficient alternative to the basic functionality of a sub-folder system.