Argosax might have been a brute, but he was still Mundus' only rival for control of the underworld. Even if it wasn't an exciting fight, I'm sure your brother can tell you what being part of it would mean to a warrior's pride.

[He remembers the honor Vergil fought with on Mallet, even under Mundus' control...]

Besides, any mission would've been less boring than sitting in some makeshift armory. With Agni and Rudra, even!