Then why do they occasionally ask aircraft to try to contact other aircraft...or check frequency readability and signal strength. (Most guys have no clue what that is and just say 5 by 5 if they can hear it.

Pilots don't know how many frequencies a controller is working, and sometimes a pilot and a controller are stepping on each other, hearing "blocked" usually indicates to other pilots to listen up to the controller and there are times the controller has no idea he is getting blocked and then gets ****ed. if nobody says "blocked" the controller will finally get a word in and then sometimes will say something to the effect of "callsign xxxx third call contact....." when in fact it's the fault of some reject racist doctor in a 182 that doesn't know when to just listen.