A spin bike doesn't give the same workout that a bike on a trainer or rollers gives. Spin bikes are fixed, so that the flywheel drives the pedals, rather than having a freewheel like an ordinary bike. It's a different workout and doesn't translate to the road or trail the same as using a trainer or rollers. I've been taking spin classes at my gym for many years. I take those classes because they're fun and a good aerobic workout, but it's not as effective a workout for outdoor cycling. Plus a trainer/rollers is a lot cheaper and doesn't take up as much space.