I was not ready to switch to Windows 8 from Windows 7 but I wanted to do WP 8 development. What I found was that even though I had a physical WP8 device and therefore didn't care about the emulator, the problem is that the WP8 tools won't install in Windows 7. You have to use Windows 8.
Now my particular hardware doesn't support the emulator so even after I upgraded to Windows 8 and installed the WP8 tools, I could not use the emulator. Works fine with an physical WP8 device.
So it has less to do with the emulator and more to do with MS simply forcing us to use Windows 8. I say forcing because there are hacks out there to get the WP8 tools successfully installed on Windows 7 (minus emulator). Those hacks essentially tricks the installer into thinking it is Windows 8 and therefore not exiting out early.
EDIT: What I am saying is that the only valid technical reason to limit WP8 development to Windows 8 is because the emulator won't work in Windows 7. However since the emulator is an optional component even in Windows 8, this shows MS is artificially limiting WP8 development to Windows 8.