Using your logic Apple should have never added Dashboard, FaceTime, Dictionary, DVD Player, Font Book, iCal, iTunes, Mail, Preview, Photo Booth, Safari, Stickies, TextEdit, Time Machine and iCloud to the OS. All of those had existing third-party solutions before they were introduced.
I think Apple shouldn't do this to developers, but it isn't just them, Microsoft does the same thing.
FaceTime and Messages are separate apps on the desktop as well, so I guess that's why. Why would you want this app to take the entire width of the screen?
- The app in fullscreen doesn’t take all the width of the screen, and you cannot adjust it like Safari. It reminds me of FaceTime, which had this awkward user interface.
- FaceTime remains installed. Messages should have it integrated completely.