[Spacepain]

Light infantry serve the core and bulk of a persons army.

Their primary purpose is to die so that other more expensive units don't die. And in turn can focus on hitting while not being hit back.

Also, secondary, they serve to finish games where encumberane would force most other units to stop fighting.

And yes for the hoarde mentality of some players (R'lyeh and Ermor) they may be almost the only thing in your army.