I am not really sure how lunging came up in this thread about bits, but I will put in my two cents in with the hope it will aid someone, especially if they find themselves spending all their time lunging their horse instead of enjoying riding.
I only lunge horses that are unstable or new before riding. Once a horse is riding well, I see no reason to lunge before mounting up. As Faye mentioned, lunging before riding can just make a horse fitter without treaching them self-control.
I have found with some horses that like to test me or try to shorten my ride in some sort of violent way, I begin by lunging them hard & long before and AFTER I ride. I make sure my ride time is very low-key and the most enjoyable part of the session.
I do this for a couple of weeks, decreasing the lunging time before the ride every day, while increasing the riding time in length and intensity, while still retaining the hard lunging after the ride. The horse learns to conserve energy and really doesn't want me to get off his/her back.
Once the horse is willing and cooperative under saddle, I decrease the time lunging after the ride too.
My goal with the method is no lunging at all, and the horse sets the pace based on his/her behavior as to how soon the lunging ends.
Done correctly, it works every time, and then I don't have to lunge them at all and can focus on riding & enjoying it.