I have never seen US airport operations shut down as a result of an aircraft in the Class D not having established communication with the tower, and it certainly does occur at many airports on a regular (i.e. several times daily) basis. What I have believe would happen is that communicating traffic would be advised of the non-communicating traffic so the pilot in command could decide what to do. The pilot in command is given that title for a reason.
Obviously high performance aircraft including airliners fly though non-radio VFR traffic regularly (in Class E airspace) so mixing traffic in that manner is not intrinsically an issue
Last edited by Silvaire1; 5th August 2013 at 17:28.