Why? The vast majority of developers never touch the kernel. If you believe a BSD kernel would get you better compatibility with other OSes, I suggest you look at the subsystem model in NT.
There's absolutely no reason for Windows to switch kernels.
Totally, totally agreed. I wish the patent situation weren't such that an interested party (e.g. me!) can't go and implement this. ZFS and DTrace (the ultimate reverse-engineering tool) on Windows would make me the happiest man on earth.