There are actually several practical issues with BCs, in addition to streamlining and preparing people for doubles.
One is that it can be quite difficult to mount a good canister light on a standard BC, in a position where the canister is accessible to turn the switch on and off. Many BCs have waistbands that don't permit sliding a canister on, so you'd have to come up with some other mounting method. Mounting a canister against the tank is not something I think a cave instructor would approve of -- it shortens the cord, and doesn't allow you to access the switch.
Going right along with that issue is the difficulty of controlling a 7' hose in a standard BC. Since you can't route it the way we normally do, under the body of the canister, you're stuck with putting the excess through the waistband, which is not very secure AND leaves a loop of hose sticking down to get caught in stalagmites you swim over.
The standard for streamlining goes rapidly up, as you begin to get into true cave.
I do believe some of the preference for a backplate system is that, if you are going to go beyond cavern (where you learn the buoyancy, the trim, the alternative propulsion and the basic team skills) they expect you want to cave dive, which is going to require doubles eventually (and ought to beyond cavern, IMHO).