I can't recall that Ubiquity asks about shortcut for switching input sources. Yes, it would make sense to me that such a question is asked for those desktop environments which need it.
Please take into consideration that standard Ubuntu does not need it. The previous default Alt+Shift caused confusion and issues. GNOME (and Unity) rely on a non-XKB mechanism by default.
I can't recall that Ubiquity asks about shortcut for switching input sources. Yes, it would make sense to me that such a question is asked for those desktop environments which need it.
Please take into consideration that standard Ubuntu does not need it. The previous default Alt+Shift caused confusion and issues. GNOME (and Unity) rely on a non-XKB mechanism by default.