The criticism is SpaceX, Tesla, and SolarCity are so tied together financially they'll all go down if one does. As an outsider I don't really care how Musk arranges his financial house. However, if I had a financial stake in SpaceX I'd be pretty upset to see my money getting invested in another company for Elon Musk's benefit.

Seems to me is that what is for Elon's benefit is to all our benefit.

That's up to SpaceX's private owners, now isn't it?

Yes it is. That was part of the postulation. I don't know how things are drawn up - they may not have had a choice.