The Kimblewick is supposed (as in - "the inventor of the bit intended it to have) a straight, bar mouthpiece, it was never intended to have a jointed mouthpiece and Alan Oliver would roll in his grave if he saw it with one! It was devised for his then little brother, Paul, to ride a fractious Pinto jumping pony called Kangaroo, and Paul needed double reins but was not up to using them (I have NO idea why I learned with double reins, but there you go- Paul is a good rider, but he could not manage double reins) so Alan "invented" a mongrel bit to act as a Pelham (which has never had a broken mouthpiece here in the land of it's birth) with roundings.
It was put on sale and went like hotcakes. It is named after the village (just down the road from me) where they lived at the time.
OK history lesson over.
Curb bits have straight mouthpieces- you may be used to broken mouthed curbs in America, here they are called "Cowboy Snaffles" (with respect, not derogatory usage, btw) but are not used and are not actually allowed in the Pony Club or in certain levels of dressage.
Jointed mouthpieces increase the pressure on the bars due to the nutcracker action of the mouthpiece. In addition, the joint angle is altered by the shank leverage to tip the bit downward and into the tongue.
So, as to their usage, as I said, Lori, we must agree to differ.