It is kind of a new idea to let students evaluate their teachers. I could not imagine such a system when I was a student. Time has passed. Personally, I believe that students should be asked to evaluate their teachers with reservations for two reasons. 
First, teachers are used to evaluate their students. That is something necessary, however, teachers might become too arrogant by always being in a position to evaluate someone. They are also human beings. I am afraid that it is possible for them to be lazy to improve themselves. 
Secondly, teachers can be innocent, still they might have some defects. It appears to be kind to tell them how students think about them. Then students can receive better education and teachers can make better teachers. It benefits both sides. 
Finally, I believe that students should be asked to evaluate their teachers for the reasons stated above. Yet we really have to be careful about the followings. It is no good that if teachers behave improperly in order to win popularity. That is totally wrong. And It is important to examine its results thoroughly. Students might be just complaining without deliberate consideration.