Why would you not just have one on there if the situation arises?
I ride western and so I associate snaffles with green horses and starting colts. Not schooled English horses. In my case, it would happen a colt might open his mouth trying to figure out the pressure and where it is coming from. Of course later in training, if done right, the likeliness of pulling a snaffle through is rare. I would rather have it than not. And it makes your outfit look complete.
I DON'T LEAD 'EM AND FEED 'EM, I RIDE 'EM AND SLIDE 'EM.