I always detach my cables when I put my headphones in my bag. When they get jostled around and squished in there, the cable joint is where the most stress is applied. I also enjoy the ability to switch out a short cable for a longer one or a coiled one as the situation calls for (such as home listening vs portable vs DJ'ing, etc). Cables can also break at the jack end, in which case being able to replace in that case is much simpler.
True there maybe be a lot of people who don't ever need a detachable cable, but there are plenty of us who do. On a mid/high priced headphone, it is a worthwhile feature and selling point.