It is almost impossible for me to get my heart rate up to a training level on the treadmill without having to increase my speed or incline to the point it hurts my knees. The elliptical gives me a much better, injury free workout.

Since I do walk outside when I can, and, of course, bike, I think it is good cross-training to do something different when I am inside. I also would just rather do the elliptical than the treadmill, and whatever you enjoy more, you are apt to do more!