Not only that, but the +1 to damage is only for the main weapon.
However, there are other advantages:
1) Two Weapon Fighting is a prereq for Two Weapon Defense. +1 to AC and Reflex is nice, especially considering that it is a shield bonus, so Armor Specialization stacks with the AC part of it and Lightning Reflexes (or Combat Anticipation) stacks with the Reflex part of it.
2) Two Weapon Fighting stacks with Weapon Focus.
3) Two Weapon Fighting is a prereq for Two Weapon Flurry. At high level, more enemies should be willing to provoke Opportunity Attacks (just due to the fact that they have so many hit points compared to weapon damage at those levels).
So sure, TWF is relatively weak in and of itself, but it allows for some other nice feats.