But isn't that situation lots of better than Android, Tizen, Harmattan and Maemo?

Core and MW are developed in the open and are open source.

And I don't see that much of benefit for Nemo if Sailfish apps would be open, as Nemo seems to move to own QML components. They still can share the middleware for example the email framework or social plugins. Just UI would be duplicate. But better a duplicate UI, as apps that don't fit into the UI/UX.

Thats where Nemo UI/UX will go: http://play.qwazix.com/grog/

Sure I can see your point that everything open would be ideal. But how should their product be successfull? Copycats would take it all.