What is the Point of the Caladbolg?

[Tabris]

It doesn't increase the damage when Tidus is at 100% HP. Only Auron's CW gets a damage increase when at max HP. All Celestial Weapons ignore the target's Defence, which is why they do more damage than other weapons, not because they're increasing the power (with the exception of Auron).

It does, actually. Wakka's and Tidus' weapon deal more damage when they're at max HP, Auron's weapon deals more damage when he is at low level (the lower his level, the more damage he does).

If I remember correctly, Yuna's and Lulu's weapons deals more damage when their MP is at max, but I am not too sure about that. Cant remember Rikku and Kimahri right now, though.