Like others, I prefer short days to complete rest days, and so do my legs for that matter. I find it just about impossible to maintain the same rhythm and technique on the bike after a rest day. That said, I do tend to take them once every two weeks or so (on average), as I become mentally tired and want to do something else for a day or so. On those days I usually hike, or if I'm in a big city, I might spend a day exploring that by public transport.