While it is true that elite bike riders intentionally let their upper bodies waste away, that is only part of the story. If bicycling's effects on legs and lungs carried over well to other activities there would be less to complain about.
Cycling is a temptress. As Mark hinted, it seems to offer great promise – in addition to being relatively easy on the joints and as challenging as you want to make it, it’s siren call also includes speed, no small amount of fun and even a potential social component. All told, a package matched by few other fitness endeavors. But when it comes to transferable fitness, alas, bicycling falls short. It is the kind of woman your mother warned you about – fast and seductive, but likely to disappoint when the going gets difficult.
The take home lesson: While driving up your heart rate might be necessary, it is not sufficient.