The reason I do one leg workouts, both at the gym (on the leg machines) and on the trainer, is trying to maintain equal strength in my "bad" leg and the other one. I injured my left leg 20 years ago, and lost some of my range of motion, and if If not careful I'll end up doing more work with the right leg. Not too big of a deal cycling, but I can sure feel the difference skiing.

Also if done correctly, one leg workouts really smooths out pedal stroke. It does not work very well on a true spin bike like a lot of gyms use. The flywheel on them tends to smooth out just about any pedaling flaws. However, on a trainer it does help.