Yup. I've heard several coaches say they would love to replace the preseason games with organized scrimmages where they can selectively target facets of the game that need work. Many of those never occur in the average random preseason game.

I can see that. The opposing coaches can decide to just say "Okay, let's take turns in a 2 minute drill scenario", "Let's work on Red Zone" etc, etc.

Yup. That's exactly. In a preseason game you might have one or two of those scenarios. And maybe for one team, but not the other.

The nice thing about those joint practices is that the other team doesn't know what you are doing, whereas when practicing against your own team the defense knows every offensive play and vice versa.