Surely it's easier to just change and wash a duvet cover at the same time as you wash the bottom sheet and the pillow cases. A duvet cover will dry a lot quicker than a duvet especially during the winter months. Also not everyone has a washing machine big enough to fit a double duvet and a 10 tog will not be warm enough for everyone during the colder months - I need a 13 tog.
If changing a duvet cover is really problematic then just do as others have suggested and put a top sheet under the duvet cover.