All conversation on the display frequency is kept to an absolute minimum to enable the pilot displaying to concentrate on his task rather than have any distractions from any source. All you usually hear is 'clear takeoff', 'downwind' and 'cleared to land'.
By keeping the display frequency as a 'need to know', we also eliminate any hazard from unauthorised/illegal transmissions (and yes this has happened in the past when we didn't keep it secret).