I do.
Yes, both many people with backyard flocks simply observing that eggshells work at least as well as oyster shells. And many university-type studies showing the same.
I've never seen any reports or studies that showed an empirical reason not to use eggshells as far as health or egg production or quality of the eggs. Some people don't want to bother with them. Some people think they need processing (crushing and/or baking or such) for various reasons; others have had no problems over many years without processing other than breaking the shell so it is not a whole egg.