It comes from the thinking that the Tom Thumb is a transition bit from a snaffle to a a curb. All it does is confuse a horse when used like a snafflle, two handed riding. The purchase digs into the face creating conflicting signals. Also it is horribly balanced, the horse needs to be on the vertical for the bit to be in the neutral position. And the straightness from the shanks cause for an immediate cue, no pre signal.
Even a broken mouthpiece with swept back shanks and a dee create a different balance and pre signal making it a slightly better version. Tom thumbs are cheap, most that don't want to spend the money on a quality bit usually don't understand how the bit works anyhow.
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