In Diablo II, the main reason people use the alternate weapon swap slots are to equip a Call to Arms Flair with a Spirit Monarch to gain access to Barbarian warcries to increase life, mana, and stamina, while the Spirit runeword increased levels of said warcries, both of which could prebuff your character for battle. Barbarians could simply use two Heart of the Oak Flails since they already had access to those warcries, and could dual wield.
I'm not sure if there are items and equipment used for similar purposes to prebuff your character in Titan Quest. Can someone explain exactly why we would need the alternate weapon slot for and how it could be useful for melee and long-range fighters or casters?