@Mark116 Well, Canadians use top sheets, as do Mexicans, and I know that there have been posts from hosts in other parts of the world here who use them as well. So it's not just an American thing.
Of course duvet covers need to be washed between guests, but for myself, personally, I use a top sheet and a duvet in cooler weather. The top sheet is folded down over the duvet, so the duvet never touches my body (which we can't assume with guests) and as it's only cold enough for a duvet for a few months here, the duvet cover does't get washed but once a season.
Ad I know that even in the US, it's the thing now for younger people to only use a duvet, with no top sheet.