They aren't that uncommon. I'm pretty sure they were on most original street Cobras (although they may have been removed), and are an option on ERA, CSX and (I think) Kirkham cars.
They're a bit that's really neither here nor there - their effectiveness is questionable and their absence doesn't usually make a difference. So as with so many other details on these cars, it's the owner's preference. I have mine off right now for access to the
oil plumbing and was thinking about putting the pushbar back on without the splitter... just to see how it looks and if it affects cooling any.
I suppose they provide some small amount of radiator protection as well - a rock bouncing up into the opening has a good chance of being slowed or deflected away from the delicate rad fins.