Addressing a couple of things here...
Up until very recently with the advent of the software defined radio (Thales Liberty, Motorola APX series) radios needed to be type accepted by the FCC. Again, up until recently, this pretty much meant that you needed to stick two or more complete radios in one box, or wired together. Of course, this made a rather large portable radio, or investing in 2x the cost. Motorola was one of the few compaines that did produce such radios. They had a dual band portable, but was rather short lived and it would have been easier to carry around a brick. Their Syntor X9000 (HHCH & Systems 9000), Spectra (HHCH) and ASTRO Spectra (also, HHCH) have been the only radio dual radios.
However, your still looking at TWO radios, and 2x the price.
Rewind many years ago, you only had one or two radio bands open, so what was the need for them? Radios pricing (adjusting for inflation) were just as expensive then, as they are now. Everyone was on lowband, then some on VHF as lowband became crowded, etc.
IIRC, when the FCC opened up T-Band, NYPD moved their due to all of its radio requirement and it was pretty much wide open to them. Fire had no such need at the time and where perfectly happy where they where. As the cost of portable radios came down and the need arose at the fire service, their fireground ops went UHF.
800 was seen as an upgrade for agencies and thats how some people got up and running there...and also because there just isn't anymore room on the other existing bands that would work well in the metro area.
Scanners are very simple (and usually ovepriced) item. They are no where complex as a two-way mobile and should in no way be compared to interop or its ease of implemenation.
MRD is used in the city, and around the state...usually as a car to car freq. All state police vehicles and many sheriff's departments use it. Some areas it is used for 911 polling or simialar function. There have been several names made up for it, and the lines are pretty blurry on which one(s) was the orginal.
Metro Radio Dispatch
Moble Radio District
Municipal Radio Dispatch
blah blah