Tom Thumbs, IMO aren't worth ever using. There are better options as far as curbs with independent side action like this bit:
Tom thumbs are not ideal for any style of riding - but if the rider feels they MUST be used they should only be used for neck reining and only by very gentle hands. All too often I see western lesson ponies packing tom thumbs with green riders hauling them around.
Personally I wouldn't use any jointed curb bit, but some people like the precision of it. I like mullen mouth curbs and low or medium port curb bits.
The shorter the shank in relation to the purchase, the quieter the bit - so I prefer mine set up like that.
A bit like this looks all around real nice to me for a neck reining horses:
High purchase, short shanks, copper mouth piece, medium port, nothing pinchy, well balanced.
I like low and medium ports as it should never reach the horse's pallet - the low one's are nice for especially touchy horses who like to pick the bit up off their bars with their tongue. Most horses would prefer pressure on their squishy tongue than their bars or pallet (which is why horses often lean into or gape their mouth with single jointed bits that are pulled too hard). So a low port allows them to do that without having constant pressure on their tongue. While a medium port would be better for a horse who takes advantage of the bit by holding it off bars with their tongue - ignoring a rider.
This is my favorite curb bit, but others have mixed reviews:
It's mullen mouth, meaning it moves away from the horse's tongue. Not so high a horse couldn't push it off it's bars - so not ideal for a savvy horse who will pick up the bit and run with it - but it's very mild. It's purchase isn't very high, but the shanks are very short. The mouth piece is sweet iron - extra yummy. What I particularly like about the shape with the rings is that it allows some 'slide' with the reins - which kind of gives the horse a minute to feel the pressure building before contact is actually made. Kind of like a pre-cue. This is my interpretation of this bit. My very few western horses all seem to really enjoy this one. :)
Just to throw in my opinion, I took western lessons for 7 years. Unknowingly riding every horse in a tom thumb. Most of the time I was neck reining.
But now I still see all those lesson horses, and ride them english. In just plain old snaffles, and they're fine.
So I'd think they were okay, as long as you have light hands.
I think a bike chain bit would be a lot worse.. o.o
I don't think they cause much permanent damage - and if a horse goes well in it, who am I to judge. At the same time if you get to decide what bit to put in their mouth Why opt for something so unclear and so strong? I always try to find the mildest option and move up if needed -or just better the training ;)
I used one on my mare for upwards of 10 years, mostly because I didn't know any better. We ran barrels, team penned, trail rode, did ring work. She worked very well in it, even with me direct reining. However, now that I know better, I would not use one. She is unsound for riding now, but the last time I rode her I rode in a Josey-Wasson mild gag bit and she liked it very much. She was not a horse that you could use a plain snaffle on.