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.
Its a fantastic feature allowing autoloader to be made safe without emptying the magazine. Also love the one touch ability to make the shotgun ready to fire. I love it and think its a grand function to have.