Well the way I use them is as "Line infantry" but only if I'm fighting certain enemies.


Lets say I'm fighting against vast hordes of unarmored levies, I use them and the Thorakitai in the first 2 lines to obliterate them before they even get close. If I'm fighting against a more decently equipped enemy I use the Thorakitai in the first line, if I'm fighting heavily armed and armored enemies I use the hoplites(whichever type I have...) in the front line, and conserve the Javelins for later use.

Peltastai are awesome, able to perform most roles reasonably well.