My last Yorkie had one dewclaw. When I finally noticed, I called the breeder and she said they just missed it. She lived 18 years with it...no problems. You just need to remember to clip that nail.
My little Zoey has both dewclaws, which I am careful to clip when I do the rest of her nails. I think they only remove them for aesthetic reasons, and maybe to eliminate the possibility of catching them on something...