Originally Posted by Omatako
...The intention of these pins is to stop the bottle screw from unscrewing itself. The good thing about the wire is that it passes from one cavity to the next so there is no way that the bottle screw can do more than a quarter of a turn because the wire goes tight. If the split pins (sorry I can't call them cotter pins) are a little small they don't achieve the primary objective - the bottle screw can turn past them.
Very good point, and it's a concern with my particular turnbuckles. The typical open turnbuckle has that crossbar at the midpoint, which allows the wire to prevent rotation of the turnbuckle:
But I have the Catalina OEM turnbuckles, which are an unplated bronze inner turnbuckle with no crossbar. I like them because I don't need rigging tape - the SS cover accomplishes that nicely. But without the crossbar, I'd be afraid that a tightly bent wire would allow the turnbuckle to rotate:
So unless I replace my turnbuckles someday, I think I need to stick with the split pins that are used currently.