A short piece of fuel line and some contact cement make good "caps" for those 'horns'.
Having experienced a halyard wrap up there once, I wouldn't consider running without them.
If you are doing regular maintenance on the swivel then it's far less likely the horns would touch the mast, but if the swivel jams even slightly the horns are the best defense against getting the halyard wrapped and broken at the masthead.
Can't really explain how I discovered this. :)
SM 209, "Adagio"
Beaufort, NC USA