I used one all the time when I was hunting geese and a flock of ducks would show up.
I had 4 buck loaded up for the geese but would want to drop down in size to #2 or 4's for the ducks.
If you don't mind shooting the same load at the birds then there is no need for one, but it sure was nice being able to block the magazine where I could manually load a different load into the chamber.