Hollywood: While your thoughts are GREAT in that light hands and soft, invisible cues are best to keep your horse sensitive and reponsive and I agree with you 100% on that, you are still missing the boat on Tom Thumbs. They are not bad bits because of the people that use them; any bit can be a bad bit if put in bad hands. Tom Thumbs are bad bits because they are poorly designed.
The Tom Thumb was initially supposed to be a transition bit between a true snaffle bit (a bit with no shanks on it, with a broken mouthpiece) and a curb bit (a bit that has shanks, and the mouthpiece is either broken or solid). A Tom Thumb technically is a curb bit, because it has shanks and leverage, although many people (and horse magazine/catalogs) incorrectly call it a snaffle bit.
Because of the way the shanks are on a Tom Thumb, when you try to direct rein you get pressure on several different areas around the horse's mouth. So if we pull the left rein to turn to the left, because the rein is attached to the bottom of a swiveling shank it makes the shank turn and tip into the left side of the horses face. This also makes the mouthpiece move with also makes the right side of the bit turn and tip into the right side of the horses face. No wonder the horse gets confused! He's getting pressure from both sides of the bit and he has no idea where to go. To make things worse, the curb stap under the horse's chin also tightens when you pull on one rein.
Otfen, the horse will resort to twisting his neck and shaking his head because he is trying to tell you that he does not understand. The majority of people, see the horse as being bad and "not listening" and punish accordingly by applying MORE rein pressure and just making this a bigger fight. He may also try to lift his head really high or really low in the air, to try to figure out what you want.
Pulling backward on the reins makes the hinged mouthpiece of the bit to collapse and jut foreward and then downward inside the horse's mouth. This puts pressure on the horse's tongue. But at the same time, the bottoms of the shanks where the reins are attached tip backward because you are pulling backward, which causes the top of the shanks to tip forward. Continuing the mess, this makes the curb strap to tighten under the horse's chin. And we are back to the same problem. The horse is getting so many different pressure cues from so many areas, that he just doesn't know what to do.
The idea behind neck reining is to be able to turn your horse by applying light pressure on his neck from the rein. To turn to the right, the rein is laid on the left side of the horse's neck. And opposite to turn to the left. When done properly, this does not include the bit at all! It should only involve the rein touching the horse's neck. In this sense when it is done correctly and the bit is not involved at all, a Tom Thumb is just fine. However, I have seen VERY FEW riders who do not pull too far when neck reining, which then engages the bit and starts all those problems we just dicussed.
Because a Tom Thumb has so many moving parts in that particular design, even the lightest pressure during neck reining will make the bit shift. Again, the shanks tip and turn, the curb strap to tightens, the mouthpiece collapses, and the horse is once again confused. Often, the horse will respond by tipping its head to the outside, or turning in the complete opposite direction that you want! Of course, most riders repremand the horse by grabbing the reins with both hands, getting angry, and then pulling directly on the reins ... which again, we already discuess is BAD.
So even if you are a truly careful rider and always properly neck rein your horse, you still have to stop or back him at some point, which is where there are problems. Yes, your horse may tolerate the poor bit action and numerous pressure point (horses are very forgiving animals), but the FACT is that that is how that bit works. And it's junk. There are SO MANY other bit choices out there -- why stick with one that nutcrackers in your horse's mouth, provides little to no release, and confuses the heck out of him? Would it really hurt anything to just use a better bit?
And just so we are clear, THIS is a true Tom Thumb bit.
http://www.valleyvet.com/group_images/26075_A.jpg
There are other versions out there that have a curved shank instead of a straight shank (true Tom Thumb) which helps the poor action of the bit a little bit ... but still not that great of choice either.