The reason it was designed this way (which I’m not trying to defend to the death btw., I concede it is a solution with some big problems) is that if you were to add a third input I don’t want the component to stop working. Having it go blank between the steps of adding an input and connecting a wire to that input seems unpleasant to me.
A better solution would be to add an identity element as persistent data to every newly added input instead of allowing it to be blank. This is something I will definitely do for GH2, but it would change the behaviour of existing components in GH1 files. So extra care needs to be taken to ensure that current files behave the same before and after this change.
Is that a solution that you can live with?