Ensuite probably adds on 'value', but are you going to be selling and when? If not, and you want a wIW, go for it. We are adding a room next to our bedroom and knocking a WIW through and everyone asks why we don't want an ensuite. We just don't. I don't want another toilet to clean thank you. I also know a few people who want to turn their ensuite into a wiw or a study/box room. So I would concentrate on what you want rather than what adds value.