I probably wouldn't have been able to write it up as nicely as George, but I had similar thoughts on the grounding bar and how it could be less than ideal for smaller wires, especially stranded ones.
That screw connection is relying on a conductor close to the size of the hole. A properly sized conductor, such as the size George points out, can then reasonably be held in place with the screw.
Mashing one (or more) smaller conductors down with the screw will probably not be ideal. You may get poor contact, and if you use more than one wire, you are probably getting to the point where you are losing the intended utility of using a barrier strip, that being you can easily remove one connection later if required.