Not that I'm aware of - the keys are like you were told - for the horse to play with. Some horses, especially mouthy youngsters and even some studs that like to fidget/chew will do well with a bit with keys because it will reduce head tossing and/or other fidgety behaviors since the horse has something in their mouth to play with. Be warned, it will cause the horse to drool - a LOT! Haha! But they are really great, soft bits for horses that are mouthy and playful! Hope it works out for you.
Bits with keys are designed for young horses. The horse fidgets with the keys in his mouth (by using his tongue) , and in result, it causes him to relax and soften. It is a good bit for decreasing the amount of times your horse fidgets, as well as helping to teaching a young horse to accept the bit.
Yes they can cause the horse to drool. But in dressage drooling or foaming at the mouth is a sign that the horse is soft and accepting the bit.
If you have any more questions or comments feel free to ask. :)