I find that hunters are loners, focusing on only themselves. They offer nothing to their teammates in combat besides bullets and grenades. Not to mention stats that prevent them from fighting on the front line with warlocks and titans, or at least not as well. Titans though, have the bubble which provides a safe place for allies and gives armor and damage boosts. Warlocks have song of light, which reduces ability cooldowns for allies nearby, while using radiance. Warlocks also make a lot of orbs for allies. This is why warlocks and titans work together well i think.