I've always thought the differences between the solid light and arrow was too minor for the average driver. I mean, look at the skills of the average driver.

In Washington state unprotected lefts weren't legal maneuvers for quite a while. They were introduced gradually starting at intersections where it would be a traffic benefit, for signaling the state adopted a blinking yellow arrow that then goes solid to signify the 'almost over' meaning of a typical yellow light.