> If you have a fan with a light controlled by separate wall switches,

This is common, but it's also not uncommon for only one circuit to be wired. The "clean" qualifier is because even if you have that line, you don't have a common DC ground.

> Unfortunately, I haven't seen this implemented anywhere.

Many Z-wave switches use the three-way traveler line for out-of-band signaling (instead of a traditional three-way switch arrangement.) (Homeseer, Inovelli, etc.) This line will communicate dimming, double taps, etc.