For my needs, the Prophet '08 makes an excellent mono synth. True, the unison modes have limited uses (and I never use them), but the single voice setting is wonderful. It sounds better than the Evolver does in mono. I would even consider getting an additional Prophet '08 to use as a dedicated mono synth, due to its more reasonable price. But it would be ideal to get the fuller rawer Prophet-6 analog tone in mono form. In fact, if it only had eight voices, rather than six, I'd probably do somewhat as you suggest and buy a P-6 Module, control it with a Prophet '08 Keyboard, and then use it in mono mode when I needed to. But with the mis-matched voice count, it wouldn't make sense, since the poly combination would be awkward.
DSI has generally done the poly-mono parallels for each of their instruments: Poly Evolver Keyboard/Mono Evolver Keyboard; Prophet '08/Mopho Keyboard and Mopho SE; Prophet 12/Pro 2. I'm only suggesting more of the same with the Prophet-6.