Since start, I have facilitated over 10 different courses. Most of the course design were developed by the others. Some course are relatively more rigidly designed e.g. SPIN – all activities are written in details in the facilitation guide, which can specify which slide to show when you go through a certain point. In addition, the TTT process is very thorough. It takes a few days and goes through almost each activity. On the other hand, there are course designs which are very flexible. TTT process is brief and the instructor guide is rather rough indeed e.g. only 1 day for a 3-day program. Time is just enough to walk through the activities, but not available at all to go through the content.
I like the former better. I believe that it is important for the participants to receive consistent messages for one single course despite using different trainers. Of course, it takes more time to develop such a course. In particular, it takes time to write a good instructor guide. It not only has to be easy to understand, but also easy to follow. It would be a challenge for the trainer to refer to the guide, slides and may be his-own note on spot in the class!! And more importantly, such courses have to stay long enough e.g. 3 years in order to be fine-tuned (and there should be course design resources to fine-tune courses). I would say this is probably the reason why SPIN is so thoroughly designed.