Con: When flying with a kid in a car seat, the person in the middle seat should be a parent. So the car seat is already moving the kid forward, and now this is moving the parent even further away.
By a few little inches? No big deal. You weren't that far away in the first place. Just my humble opinion. And yes, I've flown plenty with a kid in a car seat.
It's an interesting concept but for international I would hope that they institute some kind of recline.