I've never actually seen a studded leather jerkin or buff coat outside medieval fantasy movies or books. Do you have an example of one? People are probably confusing it with either a Coat of plates or splint armor affixed by a metal stud. The armor itself was sometimes inside the leather armor, and the affixing piece would appear on the outside.

The studs you see on the outside of the leather are rivets, used to attach metal plates to the inside of the garment. Ie this 15th century example: http://upload.wikimedia.org/wikipedia/commons/7/77/Brigandine,_Italian,_c1470,_Royal_Armoury,Leeds(internal_view).JPG

This armor, called a 'coat of plates' or 'brigandine,' was rather effective, and not nearly as heavy or cumbersome as you might imagine from the photograph.