This would certainly be helpful, it would drastically reduce the number of scenes needed to show "simple" things like lower thirds that are likely to be wanted overlaid across multiple sources
Edit: Even adding a "Show/hide source to program" hotkey option would go a long way towards making this functional. That way we could just duplicate the source across all the scenes and then show/hide it live across all of them with a single button press. This is different from current behavior because showing/hiding a source doesn't send the change live, it only does so in preview