Yeah, I really don't think it's a "Monitor channel 9 thing", but more of another general "tool" in an officers' arsenal to set up radar traps, or just have in the back ground for further info on a specific location of an accident or such.
The miniscule amount that I actually do listen to CB, I could see how a Metro/Traffic/Truck enforcement unit could get use out of monitoring 19 for getting a better location/assesment from all the pissed off rig drivers stuck behind a drive-time accident on a major Interstate running through a metro area.
And for truck enforcement units, I have seen it used most offen as a "remote PA", ---"Yes, you in the orange Schneider truck, take it to the next exit."
Rich