Eating calories back...good or bad?

To meet the goal you selected (likely 1-2 pounds of weight loss), you need to eat most of your exercise calories. Doing so is more important if your MyFitnessPal target is 1,200 calories, and less important if your target is 2,000+ calories. The exception would be if you chose an activity level that already assumes you do a significant amount of exercise (e.g., something above "sedentary" or "lightly active").

I wasn't eating much after endurance training (3-6 hr events) and ended up catching colds. Turns out this is a well-known problem when you don't re-fuel your body. Being more diligent about refueling "cured" me.

This is a common question that pops up around here. Because of that, there are a lot of good posts trying to explain why the site is designed for you to eat them back (with the caveat that gym machines and MFP may overestimate certain exercises). I like this post to help explain it: http://www.myfitnesspal.com/topics/show/818082-exercise-calories-again-wtf

peronally i dont eat my calories back from exercise, the whole point in why i work out is to burn calories and lose fat.. not so that i can eat more in a day

MFP already has a built in deficit so you are meant to eat exercise calories back. You don't have to eat all of them but if you're hungry-you can. I eat 1/2 to all back and I'm at goal now.