I have used a ball gag but in the context of "playing around in a BDSM sort of way". I would never think about having one in a disciplinary situation. I have hesitated a bit about posting this because I know that my/our view on this is not shared by many people. However, for the sake of diversity........here it is.
For me, it is important that discipline involves submission - and ONE (and only one) aspect of that means I need to not only 'get spanked' (or caned, paddled, switched....etc,etc,etc,etc) but I also have to actively 'receive' that punishment. I am expected to do something more than demonstrate passive submission. I have to make as much effort to receive the punishment properly as K makes in administering it. And in our house, 'properly' means 'with control over my physical and vocal reactions'. If I were to shout too loudly it would be almost as unacceptable as if I stood up in the middle of a caning.
It's funny really, because when I administer a spanking in spank meets or when I watch films of spankings etc., I love nothing more than to hear someone wailing! Maybe it's a fantasy of what I'd like to be allowed to do at home.
But I'm not allowed and that's that.
This is just us though - and I am aware that our relationship is very different from many others' relationships.