I know a few people that use a rack module with a controller. The only difference is the programming/sequencing capabilities. A rack module may also be alot harder to navigate at first also. When you think about it all a rack module does is provide you with MIDI capable sounds. If you JUST want sounds a module is perfect. However, most keyboards are created for the musicians that do more than just play(produce, compose, etc.)
...What are you trying to do...?