Agreed. Trills are key to developing good legato, but as you mentioned Joe, they must be performed correctly-with proper evenness and tone, or you are ingraining further bad habits. That is something a lot of students do not realize is that either you are ingraining good habits or bad, there is rarely a middle ground. So they slop through exercises and wonder why everything sounds messy. Everyone was there at one point or another in their playing, but thankfully your students have a good teacher.