The darlington transistors in the L293 are taking the role of the TIP120 that you have used previously: they take a small signal and amplify it. No additional isolation is required, as such.
However if your power-output stage is particularly risky, e.g. has moving parts near mains voltage or lots of stored energy that might blow up the L293, then you might want an opto-isolator between the arduino and the L293. Its only role is to protect the arduino (and your safety, presuming that user interface things like buttons are connected to it) if the L293 gets blown up or the load touches a dangerous voltage.
For a simple battery operated or otherwise low-voltage thing (lego motors), there's no point in using additional isolation. The L29x series are designed to be directly connected to 5V logic.