When pilots arent completely honest, it doubles the ATC workload. Not only must controllers separate airplanes, but they must also separate fact from fiction.
Like flying itself, air traffic control is complex at first glance, but below the surface its driven by basic principles. One ATC cornerstone is the need for timely, accurate information. As an air traffic controller, when I ask a question or issue an instruction to a pilot, the ol trust but verify adage kicks inbut only to a point.