Yes it is! At least that parabolic model. Is useful to minimize interference in flight in populated areas or cities with high density of WiFi equipment ... in terms of increasing distance. Mmmm .. not much help.
I think they help depending on the situation. However, the Mavic is so good as is. It's really not needed. Even on 20,000 feet it's rock solid for most
I agree the Mavic has an incredible range, but as cheap as these are and as easy as they are to install it can't hurt, kind like cupping your hands around your ears