Hidden Power is at best a base 70 attack; chances are, for any given pokemon, even if it's STABd move is resisted it will fall somewhere around 70 power. Hidden Power is thus only useful when it is supereffective or 4 X effective, thus the common use of HP Electric (to counter Gyarados, among other waters) and HP Ice (to take down any Dragons, not to mention 2X weaks). HP Dragon is outclassed in almost every way by HP Ice, and in the case of Steel types, if a base 70 unSTAB attack is your best option, you might as well switch out.