I don't use the MFP defaults, but I do factor exercise into the calculation as an activity level. Generally on MFP people recommend eating back "half" because the exercise calories that are calculated are extremely generous, so if you eat more than half you are probably eating more than what you actually burned.

You just don't want too steep of a calorie deficit. So, if you are losing slow and don't have a super steep deficit to start with, then it probably doesn't matter if you eat back any of your exercise calories or not because if you don't eat them you are still probably only at a moderate deficit. If you are starting with a steep deficit, then you probably do want to eat back exercise calories because you don't want to make an even steeper deficit. Your body does need nutrition for recovery from intense exercise. If your exercise is not intense, then it may not matter so much.

Some people also go by hunger, so if they are not hungry they don't eat their exercise calories, but if they are hungry that day, they will allow those extra calories from exercise. That kind of depends on how much you trust yourself to interpret your hunger signals.

When I exercise I only eat back 20-30% of them, not half.

I use MFP to track exercise time and type. like @tcunbeliever I've found that the default calorie estimates for all exercise here are way off. I've taken the trouble once in a while to do some heart rate monitoring and editing of the calories in the app to be closer to what I think I'm burning but...that's a lot of work I found I don't really care that much about. So, I use it to track time and type. Over the years I've also identified that I'm not hungry same day as a harder work out...I get more hungry 2 or even 3 days later so...I try to just listen to my body, and track what goes in and what I do and I don't worry about the exercise calories.