I have one that I use very occasionally with a lightly loaded Carradice Junior and a sometimes heavily loaded Sackville Medium. It was with the Junior that it fell off under way, fortunately while I was walking the bike -- or else riding very slowly -- across a busy road. It got tangled in the rear spokes and twisted out of shape; fortunately it didn't damage the spokes and I was able to twist it back into shape. It is not very stiff, which a safety benefit. (It was indeed properly installed when it fell off.) I think at least one other user had this happen to him. I've taken to holding it in place with a velcro strap.
That said, I will keep mine -- for occasional use.