We use a Macy’s Charter Club Damask collection duvet cover over our Charter Club down comforters for our king size bed (kids have them too for their twin and full beds). We have 2 different weight comforters and change them out at beginning of winter and mid-Springish. We use a flat sheet as well. I do not feel it necessary to wash the duvet cover every time we wash the sheets but I do wash it probably every other time.
It does take a little bit to put it on our king bed by myself but that’s more because of the time it takes to walk around the whole bed (which has a bench at the end) to get to the other side when putting the comforter into the cover. When my husband helps it’s very quick because each of us get to stay on our side.
I would never go back to a fiber-fill type comforter after using a down comforter. Obviously the duvet cover is not necessary - you could use the down comforter with it. But I like that it adds one extra layer of protection from dirt, etc. so that I only have to wash the actual down comforter very infrequently. It is also a way of adding color or pattern to the decor.