Well I have found everyone's responses very interesting and all bring up valid points. However, I think Thalib summed it up best.
In both dojos I've practiced in, it was common for students to 'teach' other students. Not so much as giving a formal lesson, but assisting those who may require added guidance in how to perform a technique.
Obviously the sensei is not available at every moment to everyone. As a student practicing a technique for the first times, guidance from other students have been welcome and appreciated. Others have helped me by giving me something to visualize (verbally) 'draw a line in the sand', 'carry the tray', 'bring my elbow to my ear'.
I do not agree that students should not teach other students. As a classroom teacher, the students in my classes often teach other students. Obviously they do not give the formal lesson, but during guided practice they assist one another. We learn from our models. On the mat we encounter many models (ukes)and each has something to offer. If you think you are not teaching others by simply doing the technique, you are mistaken.
I have found that I am the best teacher when I am also a student and the best student when I am also a teacher.
If someone wishes to correct another and does so with kindness and intention to help each person benefits.