I would suspect when a county is told "If you buy system XYZ and put all your people from the fire trucks all the way down to the museum janitors on it, you won't have to have separate infrastructure for all your agencies, and you're saving $", they believe it.
I wholeheartedly agree that public safety should not be on the same system as anything else. There have been so many examples of how this is a bad idea - including one I recall from somewhere in the USA about 10 years ago. The fire department chief officers used iDEN phones (Nextels) for their "chat channel" i.e. command and control establishment and operation. During a major incident, the public jammed up the cellular "lines", rendering their command channel completely inoperative.
Another example from quite near me - a local town used to have a 3 frequency MPT1327 system (1 CC, 2 voice channels) on which they had their fire department, ambulance service, bylaw, public works, and etc. There was no priority configuration (at least, that I am aware of) for the fire/EMS users. It was not uncommon for an incident where both agencies were involved (say, a car crash) for at least one agency to constantly get "bonked" when trying to use the system, because public works was nattering constantly, or worse yet, a phone patch was on so somebody could contact their wife and chat from their work truck.