I wouldn't say I don't allow tracing with any student per-say. As with many things, there is a time and place for the use of said technique which is intended to train the student/artist in certain dynamics, such as hand-eye coordination muscle memory), finesse (line or steadiness) control, pressure control (how heavily you press against a surface while drawing), and the point of articulation while drawing/painting (drawing from the wrist, elbow, a wrist support wand or your shoulder).
Within animation it's quite a widespread practice, but it's not meant to be a crutch to justify taking an effortless way of replicating subsequent illustrations. That's part of the muscle memory you're supposed to develop at an early stage of one's learning curve while learning to draw effectively and will be expected to do this effortlessly like a breathing reflex. You just do it without thinking about it. So, it really depends on your field of work/focus as to why you may need or not need to trace something.
Overall, the student or artist should be learning to draw freehand and from memory or imagination most of the time as a primary goal/skill.