A well planned lesson is a quality lesson. I have done two workshops on 'lesson planning' with my teachers, and the first thing I elicited from them is "why lesson plan?". I got them to tell me why it was important.
1. To have a structure, something to keep them on track.
2. If someone else has to take over their lesson, they are clear on what has been done, and what to do.
3. If they do the same lesson with a different group, or repeat the lesson, they have a lesson plan prepared.
4. Sharing. My teachers actually share their plans...reduces the workload.
5. To make sure that all elements are covered throughout the course: speaking, listening, reading, writing. Also to make sure that the methodology is varied, that all learning 'types' are addressed (auditory, visual, kinaesthetic, combination).
6. I also ask them to write a quick evaluation of the lesson afterwards...what went well, what they need to emphasise next time, etc.
I could go on! Each teacher has a folder for each class they take. The lesson plan goes on the top, with copies of supplementary materials underneath. Plus another copy of the supplementary material is put into a 'bank' - large files for different levels - for other teachers to access.
Some teachers do this well. Others don't. The quality lessons come from the teachers who do it!
Having said all that...I sometimes get them to NOT plan..to go into the class with no materials, either. We often find that some of our best lessons come out of our heads on the spur of the moment. But my teachers are inexperienced so this can be quite a challenge for them!
Working from 'just' coursebooks they may also lose their creativity but that's another topic!