My DH and I are arguing about this, particularly with regards to our youngest dc, who my DH says I "baby".
I don't see why they should have to clean the plate (possibly because it was an agonising experience for me as a child) and I particularly hate it when DH forces them to eat something they either don't particularly like or don't want that mealtime. It reduces them to tears, but absolutely infuriates DH when I take their side.
Had this over a chicken dinner tonight and feel as though ds2 (nearly 5) has been bullied - he was sent to bed in the end because he didn't want his chicken!!