There is a the important physiologic aspect of resistance training that is built into Spinning, that makes it very different than jumping  onto a stationary bike.

With a spin bike, you have a weighted wheel with adjustable resistance, which turns your cardio workout into a strength training workout.

Building muscle is a good thing to do if you want to get slim, building muscle mass will lower your body fat, increase your metabolism, and make you leaner. For your clients looking to lose weight in their legs, spinning will grow them by adding lean muscle mass. For most people, fat is easy to burn, where as muscle is very difficult to remove. For your clients who want to spin and lose weight, advise them to never increase the bikes resistance past 50 Percent, which will keep it more of a cardio workout. Be sure to let your clients know that Spin will never be a pure cardio exercise.