Leftovers are fine if you have planned to cook a little extra to save time and you have planned that meal. Take for example my parents who have overcooked and have a fridge full of 'leftovers'. That means that to help them not to waste food I have to eat what is there rather than a balanced diet. Therefore, it's not as healthy.
Recently dad cooked pasta, enough for 8 people and there are only 2 people who can eat it, so I have had to eat it for the last 3 days, not by choice. Also it means I can't balance my diet, if I need more protein if I am too full. Another instance is where my mother opened 2 cans of beans for my brother when he visited (he couldn't eat them all), then he left, so I had to eat them when I usually only have a couple of spoons maybe once a week. As a vegetarian it's important to mix your proteins, so eating leftovers isn't healthy when you eat food for the sake of not wasting it.