I don't see any advantage to using a zed over jumpseating. I have jumpseated while my family travelled on a zed overseas a number of times. Zeds are basically on the bottom the list anyway. Because we have young children, I am picky about which flight I choose. Even when we caught a flight with only 20 seats open, the gate agent still worked to get us all seats together.
The only times I have ever used zeds have been when an international carrier is the only option on a particular route.