it might just be a hook to induce more people to install a facebook app and/or use facebook. i don't use the facebook app myself (as you say, i don't want facebook having access to my phone), but if i did i would see no real privacy reason not to use this one too.
Do any phones (other than the Nexus line) come without a Facebook app? It was pre-installed on my Sony Ericsson, and I can't even remove it without rooting.
This 'app' though, is much more. It takes over all your home screens, and the launcher, so if Facebook were data mining on it I believe they could have information about every application you have installed, when you use them etc. To my knowledge, apps don't get to look at that sort of stuff by default.
