Richard,
I would in no way think you are the "bad guy". In fact I agree with you, almost. Most of the time there is no need for a larger busy department to listen to other another agency.
However, some exceptions would be.
(1) The University department usually scans our local PD channel. I cannot count the number of times, without our prompt, they have assisted on traffic stops, chasing down robbers, in shootings and other types of assistance only because they usually "scan" our channel and respond when close. They are a very valuable asset. At the same time it is nearly impossible to the city to monitor the University because the city is so busy.
(2) An ambulance is parked a safe distance from an active structural fire, just in case a firer fighter or someone else is injured. If they are going to stage there and not respond to any other calls why can they not scan the FD channel? In our county I don't believe they even have the fire departments channel (first issue) but even if they did it would seem good to scan that one channel and respond the moment they know they are needed rather than the FD calling dispatch who calls the ambulance dispatch who calls the ambulance and tells them where to go. Some where in all of that something is always lost and it is not just time.
(3) A few months back we had a large event in town. What went from a normal shift of 15 Officers grew to more than 30 in the cit, 20 state police and more. The city used 2 channels (which is unusual for us) just for calls for assistance and then used a third channel for special teams to respond to the events. We could not scan anything. Even the special teams who just sat and waited to be called could not scan the normal channel. What happened is that I would answer a call say an assault in progress across the street from one of the special detail units and they would have no idea why I was there or sometimes that I was even there.
Sorry for the the long stories, but all this to say that most of the time there is no need to scan but for some departments and some occasions it is vital to effective communications and safety.
Thanks for letting me vent.
Ben