I tried a commonwealth just to exercise the mechanic. What happened is that my friendly neighbors targeted the new commonwealth for attack, and my fleet was too far away to prevent invasions. Looks like all I did was create low hanging fruit for the AI. Oh, and I could locate my fleet that way because people didn't want to attack me considering my total strength, and also because I had developed good relations with enough of them.
And has been mentioned, it didn't actually seem necessary from a morale viewpoint. Whereas splitting up your empire in Stellaris is kind of a necessary evil. I don't like it in that game either, as it creates more headaches than it reduces micro, and I like it even less in that game, as I must use it.
I think there is a seed of a good idea here, but it needs massive restructuring. Definitely don't need more reasons to babysit.