I would first have to ask you what you mean by cross-training. Do you mean including both cycling and running/walking, swimming and aerobics classes, strength training and flexibility? I’m not being facetious. In my opinion, a good fitness program includes cardio, strength and flexibility training. As suggested the greater the variety of exercises you can introduce your client to, the less likely that client will become bored with a routine.
With athletes, however, the primary question is how to reduce the risk of repetitive use injury. When I was coaching collegeate nordic skiers, we would cycle in the summer and fall to maintain c-v fitness with somewhat similar movements, but allow the tissues of the shoulder and shouder girdle to recover.
I agree with many of the answers given that it’s a great way to allay the potential for boredom!