Why do some opto-isolators have that additional base pin ?
It is occasionally useful to connect something to the base pin. I sometimes connect a resistor between the base and emitter, because it helps the transistor turn off faster when the input current is removed. This matters if you are trying to pass e.g. a 25kHz PWM signal through the opto isolator.
I also assume that the transistor in the opto-isolator is not to amplify the signal, but acts more as a simple switch ?
Amplification and switching are not exclusive. A simple transistor used as a switch still performs amplification, which is why the base current used to control the transistor is lower than the collector current being switched. It is the same with an opto isolator. A phototransistor is more sensitive than a photodiode because of the amplification of current that takes place; but the transistor in an opto isolator is normally configured as a switch.
(so to use power-hungry devices like motors you would still need to connect a separate amplifying transistor (like an NPN2222) to the opto-isolator ?
Yes, because opto isolators can generally switch a few tens of mA at most.