The worth of microstepping?

[Jonathan]

Now I understand that this separates the coils but I'm not sure of the benefits as at the moment with my 4 lead motors I just use two coils and reverse the polarity. Couldn't I just join the two coils on each pole together on the 8 lead motors and use it the same as my 4 lead?

The benefit is it allows you to connect the coils in series or parallel, to get either higher current. You're not just limited to what the motor manufacturer chose.

I ask this because I have L293 Dual Half-H Bridge drivers by ST and I'm not sure how or whether the 8 lead motors would work with them?

To make a worthwhile stepper driver you need to make the H-bridge from individual MOSFETs as none of the integrated ones have high enough voltage and or current ratings to be useful on anything but a very small machine. It's still good to use the ready made ones to test the principles.