The letter opener is a poor secondary backup weapon (or even primary if you prefer stab-type knive weaponary). It is light, and in terms of durability, is worse than than the kitchen knife, the ice pick and the hunting knife. As the letter opener is one of the weapons with the 'Jaw Stab' attack, it can kill a zombie instantly if close enough and timed correctly, however a full condition letter opener only has 5 attack attempts before breaking.
It is a single attack weapon, so it is best if used in single combat or if zombies are relatively spread out when attacking.
Its tree/door damage is negligible. (2 tree, 1 door per attack). It can be scrapped for 15 metal units.