There are a lot of good reasons NOT to eat back your calories. Many people who exercise inadvertently move less throughout the day without even thinking about it, thus negating a bit of the calories burned through exercise. The body also adapts to exercise done frequently. A beginner may burn 300 calories running for 30 minutes, and only 270 calories running that same 30 minutes a few months later.
I don't eat back calories I burn unless it's a very hard workout- a long (1 hour+) run or lifting session, for example. Even then I only eat back 100-200 calories, nowhere close to what I burned... whatever number that may be
