I personally don't see the need for putting any Mac project files in the repository.
I could be wrong, so correct me if I am,
but it seems to me that the vast majority of mac users are not the type who will want to compile from source.
And if they were, they would probably know how to use their chosen compiler, and wouldn't need a prefabricated Xcode project.
If providing this makes the download bigger, even by a couple of Megabytes, I am against it.
HOWEVER, slightly off topic, I will say that based on my own experience, I think we should provide a functional mapc for mac users, as part of the binaries. I believe we'll find quite a few users who will be wanted to dabble in level design, and very few who will feel compelled to build the game itself.