The default layout - as the name suggest only single layout can be set as a default layout for a display.
Only the default layout will be displayed when there is nothing scheduled to a display.
We do recommend default layout to be something fairly simple, not often changed and to schedule more complex layout.
As you said, you can create campaigns, which essentially are grouped layouts and schedule it to your display - you can also assign/unassign layouts from the campaign at any time.
You can utilise schedule start/end time, as well as priority to display the content you want in the timeframes you want.
In upcoming 1.8 series (1.8.0-rc2 is available now) there also will be dayparting, overlay layouts, working and documented API which combined with XMR can be utilised to for example change layout on a display for specific a duration etc.