Also, if I have a planet with several ships at it, and I want to selectively launch off a few of them, I wouldn't want to have to click the planet each time...
I wondered about that after writing my post, and I thought : ejected ship could be selected if it was the last ship defending. But changing the rules of the UI could be confusing.
In any case, when shipyards come into play, our habits may change (and if ships cannot defend a shipyard, there is no point at all).