I agree - most summer films lack substance and are simply mindless entertainment. There have been a lot of great summer movies, and until I hear more I have no reason to doubt that Indy IV will be an entertaining 2 hours. What's wrong with that? What's wrong with being entertained? Have we become so high-brow and serious of a society that we can't remember how much fun it is to sit in a dark theatre in the air conditioning, with a tub of popcorn and turn off our far-too-busy brains for 2 hours?
It's harmless, man. Why not?