Comment 7 for bug 1251393

In fact Pcbnew takes in account edge cuts in footprints (at least plot files and when builds the board outlines).

The reason edge cuts are not allowed is due to:
1 - board outline must be a closed simple polygon (for 3D view, for DSN export and to have a well defined outline to make the board.
2 - holes created by edge cuts are allowed in boards, but for the same reasons, they should be simple polygons, non intersecting between them and the main outline.
Otherwise you cannot really define the board shape.
3 - Edge cuts in footprints cannot safely meet these requirement, because they are created without knowledge of the board ( grid size for instance, and other footprints put on the board).

So, allowing edge cuts in footprints is a good way to create board outline issues.