Yep, the hobby can be strange, but saving the box art ain't that strange.
Some of the paintings on the older Hasegawa and ESCI jet kits (Jaguar, RF-101, F-100D come to mind) are beautiful, and unavailable in any other format.
One of the big drawbacks in recent kits, I think, is the trend toward photos of the finished model or the real airplane on the box top. A good painting is a powerful sales draw, and they could still use the box sides for truth-in-marketing.
Frank