Personally, I have never had any need for one. However, my horses, all of them, go in a snaffle for everything we do. I don't have anything against most bits - I would like to try Bundy in a curb one day - But I simply don't need anything other than a snaffle to solve any problems i've come across.
I've got a friend that won't ride in anything but a gag..
This is very close-minded. Even if a horse hates a gag and goes wonderfully in a snaffle, she will keep them in a gag?
I beleive in an open mind when it comes to bitting. Every horse is different and will like different things. I DO believe though that you should always stick to the mildest possible bit for that horses needs - And always look at training before switching to a bit with more power.