Yes, like I said, some engines run OK with the filter on and others don't.
I started using them when I entered the hobby 5 years ago, to help to keep the grass clippings and dirt out when my not so perfect landings went astray
. I got sick and tired of picking small pieces of grass and bugs out of the carb, so the filter seemed to be a wise choice back then. I only use the course screen filter when using one to allow for maximum airflow.
I don't use them on any of my new additions to the fleet, landings have gotten better,
but they still remain on the ones that I originally put them on, as they are doing no harm on those.
Cheers
Curt