Many moons ago — when I was younger, fitter and slimmer — I was lucky enough to attend a martial arts seminar with a famous martial arts master. His attitude and practice was anything but the mystical tosh spouted by some of his movie equivalents. Rather, his focus was intensely pragmatic: he had studied martial arts all over the world in order to find out what worked. This focus had been started by his early U.S. “police judo” training: if someone comes at you like this then you try this or this or this.
The reason I mention this that many teachers seem to hate the idea of a scripted lesson. To my mind, a scripted lesson doesn’t necessarily entail a series of reductive, robotic responses. Instead, it could simply be a list of suggested sequences and interactions that have been found to work historically.
I often — perhaps too often — write and use my own resources (“Use another teacher’s resources? Ugh! I’d rather use their toothbrush…! “)
But even I, the king of the animated PowerPoint and the Amadeus of the well-crafted worksheet, would consider using an externally written script if it demonstrably worked.
Whether Direct Instruction scripts really, really work in this sense, I can’t say. The proof of the pudding will be in the eating, or in the teaching.
And, as any actor could tell you, even the simplest script can still leave the actor with a wealth of choices…
e=mc2andallthat 
“would consider using an externally written script if it demonstrably worked.”
You particularly would only know that it worked if you had already tried it out, and wanted it to work.
I think the army makes much more use of scripted lessons, designed by specialist instructional designers. Not saying that the model would transfer directly to school – but I agree that it does show that the general approach cannot be entirely dismissed. Crispin.