(I’m not using teuxdeux anymore. And after a short liaison with Todoist, I’m back with OmniFocus.)
Reading the old scripture on the forecast perspective makes it clear to me, that this was intended to be used as a way to tell how due dates and events on a calendar interact.
It’s true you can also show deferred tasks and use a forecast tag to surface tasks without a due date in this perspective, but that is just secondary functionality. And it shows.