I'll admit that if you are monitoring a relatively inactive Motorola system, you might be able to "get by" with a conventional scanner. A friend of mine in Florida was able to do this a few years ago (monitoring is local sheriff), but when they started loading other services on to the system, it became increasingly difficult to follow the conversation. And, you can forget using a conventional scanner if you are trying to monitor EDACS systems! BEEP, BEEP, BEEP, BEEP.....
Analog Trunktracking scanners have become very reasonably priced in recent years. I am sure the digital models will also become cheaper down the road. 8)