I've seen pros use white cards
but more for fill in light -
I guess their set up is usually for soft light
and to eliminate those dreaded reflections.
Using a moveable white card to eliminate reflections is real inventive.
I also kind of do that by using a white board in front of myself
and shooting through the gap between that and the tilting back board.
But now I mostly don't - since it is cumbersome
and reflections can be eliminated using just the tilting back board.
(I shoot mostly hand-held dSLR
as I find it easier and more flexible to find the best angles through the viewfinder)
But like Coop, I can't seem to be able to shoot straight-on
since mirror finishes will reflect the photographer/camera.