Yes, I know that, but the keyboard-module combination creates a superior 12-voice stereo field that doesn't require modulation, and has other advantages as well. It's an ideal in the process of designing sounds to have individual units in front of you. It also allows you to differently process each instrument, which deepens the field even more. For example, it's quite a stunning effect to use a faint sound with lots of reverb as a sort of musical backdrop to the main sound. I suppose this is why Dave bothered to add the B ouputs.
I haven't played a Prophet 12 myself, but I've listened over and over to all the P12 recordings on YouTube and Soundcloud, and not one has struck me for its stereo depth or even resembled my idea. I usually hear panning, which is not at all what I have in mind. The above recording is another example of this. I'm after a natural sounding stereo/spatial effect - a wideness and a depth - and an instrument jumping from side to side is anything but natural. To me, it's actually distracting. And although it's true - pairing an instrument and a module still leaves you with the same instrument - nevertheless, it can unexpectedly improve the overall tone. That's what I've found with the Poly Evolver and Prophet '08.
I've also experimented with three MIDI-ed Prophet '08s. The master is centered, while the slaves are panned to opposite sides. I compared the results with two Prophet '08s panned and was quite surprised by the results. Even monophonic patches sounded far superior.