They’re scripted on your side, but not on the other team’s side, just like parts of games are. And more importantly they’re scripted with things you know you want various looks at, which is productive. In preseason games it’s great if a backup RB hits the crease and goes 60 yards, but there goes an entire series of offensive information you could’ve gotten. In joint practices you reset and go again, moving onto the next thing you want to get a look at without wasting time or losing snaps. Plus you usually get an extended look at starters vs starters, 1vs 2’s and 2 vs 1’s, etc. in structured joint practices, which are all valuable reps and that tape is gold. In a preseason game, you’re getting much less direct work with worthy opponents across the ball, in general.
I’m not 100% either way but I definitely see where the argument comes from that the joint practices are more important and give more information. At least for 1st and 2nd string. For depth, it’s probable the preseason games give more to evaluate. Different purposes and all that