A couple of reasons to track your miles - 1) if you are going on an organized tour or ride, you need to be able to compare your fitness against the tour/ride company's measurements. If the brochure says X miles of technical terrain, you better know if you can ride X miles of technical terrain. 2) If you want to share routes with friends or use in another sport. Because we knew the miles from our house to a local trailhead via bike, we knew that we could translate that to XC skiing. The use of miles for measurement is the common denominator for guidebooks, maps, trip brochures, etc. If you do none of the above, time-only is fine. I also tend to pay attention to miles so if I need assistance from law enforcement or a friend, I can say "I'm X miles" from such and such.

Last winter I went to a panel discussion with Chris Horner, Adam Craig, Carl Decker, Chrissy Ruiter and Megan Elliot. It was interesting to hear them talk about how they train. For the mountain bikers, they measure in time as in "races are no longer than X hours" so they train based on time. The women road racers talk about time and miles, but their road races are shorter than the men's and about the same time as the mtb races. Chris Horner talked about miles, but then he trains for 2,100= miles in 20-some days, a whole 'nother world as far as most of us are concerned.