Johore is a good colony to have because of its tropical wood and eventually rubber, and the precious metal is just a nice addition in my mind. It may not be helpful now, but if you manage to turn Johore into a full-fledged state and manage to increase its population significantly, those provinces will be far more valuable. As of such, however, it's not really that helpful to have precious metal provinces. It can provide an immigration boost tin the case of an event spawn, but that isn't the case in Johore.