There's a few possible reasons to choose the MacBook over the MacBook Pro, if you have the money for an MBP:
1) Keyboard. A preference thing, mainly, but if you prefer the MB keyboard, it would be a selling point over the MBP.
2) Smaller size. The MB is smaller than the MBP, and for someone who doesn't need the extras of the MBP, the MB would have some appeal on size alone.
3) Easily replaceable hard drive. This is conceivably something more of interest to pros than consumers, but at least as of now, it is much easier to swap out a MB hard drive.
4) Durability. The iBook/MacBook design is aimed at durability, and perhaps less "delicate" than the MBP. Not to say the MBP isn't durable either, but just in different ways. A good case of course makes that kind of irrelevant anyway.
Probably some other things as well, but these are the ones that come to my mind.