The damage per second (DPS) this adds to a weapon is the 2 damage divided by the weapon speed of your weapon. It normally takes 14 attack power (AP) to add 1 DPS to your weapon.
To calculate the attack AP this enchant is worth, take the 2 weapon damage, divide it by the weapon speed of your weapon, and multiply it by 14.
1.0 speed (cat druid): 2 / 1.0 * 14 = 28 AP
1.6 speed (tank weapon): 2 / 1.6 * 14 = 17.5 AP
1.8 speed (dagger rogue): 2 / 1.8 * 14 = 15.56 AP
2.5 speed (bear druid): 2 / 2.5 * 14 = 11.2 AP
2.6 speed (sinister strike rogue): 2 / 2.6 * 14 = 10.77 AP
3.6 speed (mortal strike warrior): 2 / 3.6 * 14 = 7.77 AP
On two rings this can offer a significant boost that no non-enchanters can obtain. However in many cases it is not quite as good as actual attack power, strength or agility. Any special abilities or buffs that depend on attack power or stats do not benefit from it (bloodthirst, shamanistic rage, unleashed rage, blessing of kings, heart of the wild, vitality, and many more.)