Example of changing form benefiting performance:
Physio videos an athlete notices that the runner's arm is coming too far across his body, and the upper thigh breaks inward. The assesment is that there is a weakness in the lower back and glutes. Exercises are given to this effect, strengthening the gluts, hams and lower back. Later on, form has changed: the arms are straighter and so are the legs, as a result of an increase in core strength.
The reason Rosa says not to mess with form is that it is a symptom, not a cause. The cause is usually a weakness somewhere else in the body. If you address that weakness, the form will adapt.
Doing drills is probably a good way to teach good form, but if you don't correct the weaknesses, it won't do much good. The same is true of the POSE method: you might benefit, but you are only really covering up weaknesses. You have to look at the root of the problem. Paula Radcliffe gets an hour+ of physio a day, so she's addressing those issues. What she looks like when she runs is best for her body.