According to Vetron's Guide to Weapons and Armour, the main gauche has a "2" effectiveness rating.
Main gauches first appeared in Ultima V, at the same time when dual-wielding was first introduced into the Ultima combat system. Thus, combatants could choose between wielding a single weapon, a weapon and shield, or two weapons. The weapon has remained since.
Daggers offer two choices. The common dagger is useful when its ability to be concealed is a benefit; and it can be thrown in open combat. The main gauche is more formidable, especially when used as a second weapon.
Similar to a dagger, the main gauche also sports defensive hand protectors that can be used to parry on opponent's attack. Unlike a dagger, a main gauche cannot be thrown.