A horse that accepts it and is comfortable with it. A well trained horse will go just as well and just as supple in a snaffle as he would in a Pelham. Horses prefer different bits to hold in their mouths since all of their mouths are different. Find the one that your horse prefers and is happiest in.
I've always been taught that it is a great tool for a strong horse or one learning harder maneuvers. The snaffle connection is pretty straight forward and is the rein you have the most connection with then the shank is there to engage as a back up or in combination. The shank adds a bit of leverage with the pole pressure which causes a more round horse, the curb chain also adds a bit of "bite" to it so the bits will engage then the chain will if the horses doesn't respond just to the bit or also in combination (all depending on how tight the chain is)
With the two rein set up, I can use the snaffle rein as needed and have the curb rein as needed. I will often put a pelham, with the mouthpiece appropriate to the shape of an individual's mouth, on a horse that I get in for rehab until I know how it goes.
I've used them on youngsters that will be neck-reined as a goal.
I think they can be a very useful bit to have in ones collection, I usually have a mullen mouth pelham around somewhere just in case :)