I find them useful (Panders, anyway) as skirmishers. In defense you can use them to disrupt an enemy advance: place a few units of light infantry here or there in front of the advancing enemy line, forcing some of them to stop and fire while the others either keep advancing or turn to fire at the skirmishers as well. Either way, you've disrupted the enemy line, and created holes you can exploit. You can also use them to distract enemy units and often pull them away from the main fight; basically, your 400-some gold unit of 60 skirmishers pulls their 700+ gold unit of teched up line infantry off on a wild goose chase for a while, which can only level the odds of the main fight or tilt them in your favor. Later you can finish off the distracted, isolated units in detail.

Lots of uses for the different light units. You just have to be creative and get away from the line up and plug away at each other mindset.