To avoid the coupling issue I assume. I'd understood that coupling was more of an issue as the distance increased along an axis say. In the case of an axial thing are pretty close together. Still a concern though, huh? We still can do it separately, so I'm glad to have the input.
Now in your case the magnet rotor speed is also alot higher. This is also a factor? In our case we're direct-drive.
~ks