I don't think there is any problem with a highly constrictive script if it works. Again, the teacher might be bored, but the purpose of school is educating the students, not entertaining teachers [1].

As for students advanced enough to make connections between metaphors in various literary works, go ahead and get off script. In fact, the few examples of DI scripts that I've seen do allow this - they tend to be flow charts, not screenplays. They instruct the teacher to skip/redo sections based on student performance on standardized diagnostic quizzes (i.e., go ahead and teach metaphors if they understand the use of commas.)

[1] Side note. I believe that "entertaining teachers" is why we tend to have so many literature classes, and a dearth of classes on factual writing. Teaching students to distill a spreadsheet+employee status reports into a department status report using only simple language is boring, but incredibly useful. I developed a lots of bad writing habits while an undergrad, which increased page count and obscured the vacuousness of my literary opinions. It took my PhD advisers about 2 years to beat those habits out of me.