Agreed, Don.
The Code should have been written in a more positive manner, rather than excluding anything as generic as a sheet metal screw. Any screw that is screwed into sheet metal could be construed as a sheet metal screw.
I think, though that if an inspector called me on having a good, solid machine screw used as a ground, and wouldn't let me off with 90.4, I would be inclined to whip out my box of crummy Ground Clips, show him the little UL stamp on each one, and exclaim "How do you like me now?" I think this would make an inferior connection as opposed to the first one.
I see on a daily basis Tek screws, toggle bolts, plastic anchors, drywall screws, and the like with ground wires shoved under them. I think the Code's intent is to provide a solid termination that won't come loose, and is used solely for the purpose of grounding.