I'm doing much more mountain biking as cross-training this year. I think it does use portions of the same muscles, but even more importantly, it adds to general fitness, fat loss, and cardiovascular performance. Also, doing lots of cross training allows you the opportunity to remove a specific activity if you need to due to injuries (or boredom, or weather) and still maintain your fitness level. I speak from personal experience on that one as I had to take 6 weeks completely off of running, but with all the cross-training was able to run a 25K, 50K and 3:19 marathon within a month of returning to running.
