Yes HR is used to measure decoupling in aerobic rides, but HR gives you another view into what your body is doing, eg. You can actually track progress and fatigue on a daily basis, you can also see what parts of each ride you work harder to produce the same wattage (weak areas), and wattage vs Hr on specific climbs on different days, etc. As well as if say, you are doing 1 min intervals, obviously your heart rate is going to climb the whole min, but you can actual look at where it climbs quicker at a constant wattage (and also where wattage drops). If your HR climbs after X min, etc. It is also more accurate to use HR than using power for aerobic work. Just measuring specific CP zones gives you a very "narrow" view, a 20 min test (or your FTP) only tells you how strong you are at 20min or 60min. "Training and racing with a power meter" and "The cyclist training bible" are good books to read. There are also a lot of good articals on the training peaks site, as well as the wattage forums.
