I heard from people in France and Bulgaria that people sometimes use honey to sweeten coffee (mostly older people).
Is it common in some cultures to use honey? Or is it just some individuals doing it?
I confirm MT San's point on availability. Sugar has become a commodity, but in the past, it was a luxury item. This luxury trait remains with high-end cane sugar, etc.
In the past people either dos not know about powder sugar, or could not afford it. They used instead honey, maple syrup, or other locally (so cheaper) available sweeteners. This habit remains for some people, usually older, as they have started in their youth, and that became their personal standard.
Interestingly honey can be more expensive than sugar today. Sugar is now a major commodity, and the price is really well "engineered". Honey has lower circulation, and the prices are comparatively high. But now the Elderly may have better incomes too.
By the way, no citation here. This is personal observation of my relatives, and a parallel with chicory, which originally replaced coffee in "poorer" regions of some countries, and the custom remains, even though the regions may not be "poorer" anymore.
Please see this story as an example. I assume, it was common when refined sugar was less accessible.
Still, I believe it is a personal choice and as valid as any other flavor such as caramel or chocolate. I have encountered a few (but not so many) people who prefer their coffee with honey instead of other sweeteners.