Lastly, "Is Apple planning native PE execution within OSX?" - if they were _planning_ that, they wouldn't include this into a production release of the OS. This means that it's already used for something.

That, or they implemented the support in such a way that conditional compilation (using #ifdefs) was significantly more complicated and bug-prone than just leaving it in and not documenting it. If you've ever written a C program that had to behave differently on different platforms, you've doubtlessly run into this issue once or twice.