It's the iOS paradigm to make feature simple standalone apps. Why do you think Apple has both a "Photo Album" and "Camera" app? And both a "Phone" and "Contacts" app?
Many users upload photos to Facebook, so there's a huge incentive to make an app that makes it simple to do this. Even if the main Facebook app has feature parity with FBPhotos, it's still going to come with all of the cruft.
Many users upload photos to Facebook, so there's a huge incentive to make an app that makes it simple to do this. Even if the main Facebook app has feature parity with FBPhotos, it's still going to come with all of the cruft.