You won’t get a bad service if you don’t tip and in my student time I rarely tipped, but never had a bad experience with this. Now that I have the money, I usually have a batch of 5€ and 10€ banknotes with me, specifically for tips. I try to tip around 10% and the people usually are really grateful.
Physically I agree with you, but psychologically it is much easier to control what I eat when I regularly go to the gym because when I work out and eat too much it feels like I am wasting my time there. So even if I only burn 300 kcal, I feel like I am taking care of my body and pay more attention to what I eat over the day.