It's really relative to what the actual prescription is and also the preference of the patient. If the Rx is approaching a high value such as -4.00 and above then it's generally time to consider high index. Also, they are more aesthetically appealing than other materials, such as polycarbonate, which would be thicker and more unseemly given the same Rx. Also, high index has a higher abbe value and what this means is light refracts more cleanly through the lens making the vision clearer. All in all, higher index lenses are worth it if applicable.