Assigning homework between lessons I think is essential to the learning process. However the homework I assign typically isn't like school homework and problems. Instead my homework is to always be thinking of (and writing down) questions to ask me the next time we meet. Helping a student is a lot more successful when they come to me with specific questions and specific problems that we can address in the tutoring session. If the student does this I think the tutoring session more worthwhile because it is focused more on solving the issue than finding it.
Sometimes if I see a specific error...