Agree with Silvestris, especially the hands-free part...and more specifically in situations where turkeys are close enough to be able to see the movement needed to work a box or pot (or any other type of call that requires movement that might be picked up by a wary gobbler).
Mouth calls (and air-operated calls, in general) have a much steeper learning curve in the process of being able to make realistic turkey sounds. It is not advisable to attempt to use a mouth call if someone has not developed some modicum of proficiency in using one. I can't tell you how many people I have seen trying to use a mouth call while hunting that would have been much better off just sticking with a box or pot...and learning to minimize movement in close-range situations.
There are ways of setting up on turkeys that will allow the movement needed to use a hand-operated call...even when turkeys are pretty close...and even when not using a blind of some sort. Those that use boxes, pots, and the like should practice using those calls one-handed (if possible) and in odd positions...for instance, like laying on the ground next to you in your set-up.
When setting up on a gobbler (sitting against a tree, for example), it is always a good idea to angle your body a bit one way or the other, rather than straight-on to the gobbler, to take advantage of your natural ability to swing your gun based on whether you are shooting right or left-handed. Doing this also allows you to obscure the movement needed to operate a call, to a degree, with your body positioning. Even though I use mouth calls, I often will set a pot and striker on the ground next to me in a set-up such that I can reach down with one hand, my body blocking that movement, and scratch out a few different-sounding soft notes if I think they are needed ...which often enough, it seems they are.
Bottom line for me is that I encourage all turkey hunters to take the time to learn how to use a mouth call with some level of proficiency. There will definitely be times when it is an advantage...but that in no way infers that those that do not want to do that cannot get along just fine without doing so.
