For one, shooters are distinctly inferior to melee-based characters in the game (even if not as bad as in the first game). For two, dual-pistol shooters are better than rifle-users. HK loses out on both counts. He's not a patch on en-Jedi'd Mira or Atton as a shooter. He also can't whip out the healing/inspiration stuff whenever needed, or use a lightsaber when the enemy gets too close.