I think that twitches are a useful tool, but only when used properly, of course. Once, one of or ex ex farriers used a twitch on our paint mare that is already touchy about her feet and hates being tied. So,she likes to already rear back if she doesnt like the person handling her feet. We were trying the guy out, and the first and second time he came out he did fine with our horses, but then he just seemed to get really impatient and when she didnt want to stand still, he got p****d and went right for the twitch without even asking us. I was really angry when I heard about it (only my mom was there) Luckily, my mom said to stop, of course. We stopped using him after that.
So, what I bealive is this: the person that owns the horse should know it just as well as the farrier. When to use the twitch, when not to. If the horse can't handle it, then don't push it. Work with your horse if it becomes dangerous with the farrier, so a twich isnt needed. I don't think it should be used everytime. Once or twice, but no more then that. If there is a situation where the twitch is required, then you should know that your horse has some issues to be worked on when it comes to handling having it's feet trimmed and shod even. Just my two cents