However, as a general rule, they should not be used, especially as a "transition" bit or on green horses. Most horses will gape or toss their head if you use the bit to direct rein (pully rein) or pull on it too hard to stop.
I just want to say here... from my experience a horse will gape or toss their head if you direct rein or pull too hard on MOST curb bits, especially those with high ports.
I really don't see a lot of difference between a curb bit with a jointed mouthpiece and a curb bit with a ported mouthpiece. Both bits are going to hit the roof of the horses mouth when pressure is applied. Ports provide tongue relief but they also add palate pressure. On horses with low palates, ported bits are like a torture device!!
I'm with Skippy!, a bit is definately only as harsh as the hands.
Also i'm nto sure who said it, but yes there are plenty of english bits that combine a jointed mouthpiece with a leverage effect. For example the dutch gag:
I see this bit used a TON in showjumping and even PC circles. It uses nearly the same action as a TT, but has nowhere near the same stigma attatched.
Personally, I don't ride any of my horses in anything more than a snaffle. Never have, with one exception, a horse I had who had been abused and had huge bit issues, on whom I used a ported kimberwick with no slots. (which was necessary, he worried over the snaffle so much he got his tongue over the bit and cut his mouth up) But I also have no need for the action of a curb bit. I ride with direct rein contact and neck reining.