I am going to put in my 10 cents worth, not as a high class chef (which I am not), but as Mr Joe Average.
I know that when I cook for other people, I like to see empty plates around the table. This makes me feel content that I cooked a good meal. Sometimes you my give someone too much food (example - they are on a diet), but that is at home.
So i could easily see chefs (of any standing) having the same point of view. What better way for a patron to show the chef that the food is good, but by eating everything on the plate. A long time friend of mine is a chef at a good, but not 5 star restaurant and he agreed with me when I asked him this question. He said it is the best way to see if the patrons like everything he is cooking, otherwise where to improve or change.
Personally, every time I Have been to a high end restaurant (HER), I find that there is never enough food on the plate. Not counting the carbs like potato, pasta, rice, et cetera of which at least one is also with every meal, we normally have between 8 and 10 different vegies with some meat on every dinner. At HER’s I normally find that there is about 2 different veggies on main course and the piece of meat is quite small. That last HER I went to, the main course had 4 large potato chips (about twice the size of a normal potato chip at about 3/4″ inch across and 4″ long), 2 small length wise slices of carrot and 1 small piece of broccoli with the meat only about 3″ round. It looked fancy all stacked nice with little swirls of sauce on the plate, but to me, it looked like a child’s meal as there was so little on the plate. I find myself thinking - “Is this a appetiser or the main meal?” As usual, as soon as I got home, I raided the fridge as I was still very hungry. Now I am less than 90k and 186cm tall, so fairly slim, and find HER’s very poor value for money because I always come away feeling hungry. Much prefer to go to a local club’s dining room where I can get a good sized and nice tasting meal at a reasonable price.
And then there was my grandmothers (all my grandparents and my mother grew up on farms), both of whom would be worried if you ever left any food on the plate. They never seemed to think that someone didn’t like something, instead they were concerned with your health - “Are you feeling OK”, “Are you sick” would be the first questions they would ask. So I suppose I am biased as it was expected of us to finish everything on the plate, even if we didn’t like it (example - Brussel sprouts are not my favourite veggies ).
The conclusion for me is that given the high cost and how little is normally on the plate served up at HER’s, I will always clean the plate. But I tend to avoid HER’s unless I have to.
Besides, as others here have mentioned, there is so much food wasted from restaurants that in the odd situation where you do have trouble finishing the meal, ask for a doggy bag, even if it will only be given to the dog.