The Thunder Spear does the same amount of damage as the Defender, and like that sword, has some nice extra effects that make it worth keeping. Instead of evasion, though, the Thunder Spear has (as its name implies) some Lightning-based properties. It's strong against foes weak to Lightning, but more importantly, can cast Thunderbolt 16 on all your enemies as much as you like. It's not a bad Ability, especially against enemies weak to Lightning, and being unlimited is just icing on the cake.