I was always taught that you should never take a horse out on trail with anything less than a curb, but then again, that you should never direct rein with a curb either.
Several others have already made the point, but I had to jump in on this one.
One way to know a statement in incorrect, if it is uses the words "always" or "never." Nothing in life is 100%. This is even more true with horses.
There may be some horses that should not go out on trail with anything less than a curb. There are also other horses that should probably not be on trail WITH a curb. It really depends on the horse and the training.
I found it interesting because I do not ride horse on trail with a curb. I usually use a bit less bridle or an occasional snaffle. However, when we are competing (speed competitions in a ring) I stick with a particular curb bit. I guess that makes my horse the opposite of what you were told.
I am not a fan of Tom Thumbs either. However, if it works for you and your horse (meaning you are using it properly and it is not acting harshly on the horse's mouth) then go for it. Depending on the horse (and you), it may also be possible to train/transition that horse to a different piece of equipment (hackamore, bitless, halter, snaffle, different curb bit, etc.). Pretty much depends on what you are looking to do and if you are getting all the "performance" you need/want from your current equipment.
Don't go with what people tell you is "right." Go with what works best for you and your horse.