Are these 12v power supplies worth using? Is additional circuitry needed to drop voltage to meet USB standards?
I have an external HDD power supply that is almost identical to yours (it has the same model number, but a different brand name). One notable difference is that when unloaded my PSU puts out a more reasonable 5.13V and 12.6V.



This type of switch-mode supply rectifies the AC mains to produce DC, then chops it up into a high frequency square wave which is fed into a ferrite-cored transformer. For multiple output voltages the transformer can have several windings with different turns ratios. 

Output voltage is regulated by varying the on/off ratio of the square wave, but this affects all outputs so putting more load on one output will cause the other output's voltage to rise. The PSU cannot regulate each output independently, so it may be designed to stabilize the voltage of only one output (usually +5V because this is more critical for digital logic), or take an average of both. 

Switch-mode supplies often need a minimum load to get good regulation. Cheaper units may have no regulation at all, simply relying on the transformer ratio to set the output voltage at the rated current. Your PSU may have worked OK with a hard drive because it is matched to that load - or it might be faulty. Either way I would not use it with the Pi, which could be damaged by anything over 6V. However you could use it to power a suitably rated step-down voltage regulator (AKA DC-DC converter).