I agree with you that it's easy to be too rigid but that would only happen IF you follow through a lesson plan word for word. From my teaching experience, about 90% of the time I had to adapt the lesson plan to suit the students' mood for that certain day. It's almost impossible to expect that the students will do what you want them to do. After all its not called a 'plan' for nothing...
If a lesson gets too boring, ANY teacher would do what you would do (which is to change the pace or maybe alternate the activities with some others) because it would be pointless to follow through with something that no one is interested in the first place now wouldn't it?
Now that's my two dollars worth.
