You can absolutely withdraw from another players structures. I'm pretty sure you can transfer into other players creeps / structures. I think you can heal another players creeps, and I'm pretty sure you can repair another players structures. The only one really missing is upgradeController. I agree I think there is no reason to prevent it since it would allow novel behaviors of players working together. I've tried to do this in the past but in the end we settled for me dumping a pile of energy right next to the controller so they could upgrade it themselves.