Depends on how you use them. They are a big part of those bodypump classes and I've used them as part of my powerlifting regimen. The bands by themselves may not develop much in the way of muscle.
I'm sure qt or someone w/ more PL background can speak more to it. Depends on what you use with them and what you are expecting from them - they are great as a part of a larger training regimen. By themselves, eh. Maybe help you stay limber just like the other parts of pilates, etc.