I've neglected to post much on topics like these in recent years largely because Texas' general decline in football has brought out the absolute worst quality of discourse. Nonetheless, this is a more general topic not specific to the Horns, so here it goes:
1. As has been already stated, the first 15 play script serves two purposes: it eliminates the guesswork, and all baggage that comes with guesswork, by the offensive players and allows them to run plays with which they're most familiar AND it also gives the HC and OC an idea about how the opposition is going to defend certain plays. So let's say that the offense runs the much maligned bubble screen to a slot receiver out of a trips set. Let's also say that this is on the first play of the game and the playside CB shoots the screen right off the snap. Ok, they've clearly been coached to do that...probably because the offense has shown nothing to counter that, at least not from that particular formation. An OC can dial up something a bit later in the game: call the bubble again, but this time have both the X and the other blocking slot WR fake like they're gonna block on the first 2-3 steps and then shoot up the field. Well the CB isn't gonna be able to defend the X because he's been told to flatly ignore him. The other slot can run a seam/post and it serves no purpose other than to occupy the playside safety (although if that safety is stupid enough to over-rotate to defend the X running a fly pattern, obviously that slot is very likely to be open downfield in the middle).
2. I agree with CTJ's characterization of the baseball analogy.
3. The script has contingencies. Have you seen those play calling charts that the play caller holds in front of his face so as to conceal his lips from the camera? Yeah, they're the same size up in the booth. Furthermore, the script on that chart constitutes a very limited fraction of its space. There are other spaces that list contingent play calls that take other factors into consideration that normal scripts do not. So, for example, it'll say what to do if the defense isn't showing the kinds of looks they've been known to show. Maybe they're blitzing on first down. Maybe they're using a 4-man front even though they're usually a 3-man front defense. Maybe they're using a single high safety instead of a cover 2. Maybe they're showing zone when they usually play an aggressive press. The contingency plays also allow for the OC to bail on the script because the offense has found itself in a predicament where the next play call wouldn't be helpful. So like maybe it's 3rd and 7 on the 5th play of the drive and you've got a power dive called for that one. Just bail on it. Duh. They've got scripts, but they're not abject slaves to them. In other words, if you took every coaches' scripts and you read through them and matched them on the film (and this all assumes you understand the names of the plays compared to what's being run), you will very often find yourself saying "hey, you didn't run the play that was in the script on that one! You're a great big phony!"
5. As to the defense having scripts, it's already been said. It's a much more reactive concept and a script can't help with that. So say you're a 4-3 defense and you trot out your regular personnel even though the offense is coming out in a 5-wide set. You're already at a disadvantage. And let's say you're doing an all-out blitz. Ok, now you're pretty much fucked unless the other team just sucks that bad. Offenses are the ones with the scripts because they're the ones with the ball.