First of all, letting your teacher know how well he/she teaches them, helps them to realise what works and what doesn't. For example,if the teacher thinks that a specific learning skill is working, but majority of the students don't share the same opinion, they should be able to let their teacher know.
Secondly, students should state their opinion because their may be a teacher who is not teaching well and students are uncomfortable with that particular teacher. But, just because kids do not like their teacher as a person doesn't mean that that should effect the evaluation, the response should be based on the teachings.
Lastly, if there are very good teachers, and the students agree with this, that would boost the teachers confidence because they could teach knowing that the students appreciate him/her.