I use components always. They could be groups.
The process I used is to cut the rectangle to the clipboard, then open the component for editing and paste the rectangle in place. After selecting all of the geometry and using Intersect Faces, I erased the parts of the rectangle that stick up above the molding but this leaves the rest of the rectangle in place as the mitered face of the molding. I just made sure the face wasn't reversed at the end.
If you don't paste the rectangle into the component/group and just intersect the molding with model, the miter lines will be created but no face. You could use the Line tool to trace an edge to form the face if you want. That's not so bad for a flat mitered face but it would be a lot of work for a coped molding end as you might have with some moldings.