[Taqwus]

Graeme --
In this case, the choice was between casting useless spells or melee, because in the arena it's victory or death (unless, perhaps, you can Return out. Hm, if it's a one-round tournament, can you summon Lammashastas and return out, and still win? Hmmm.) If he has no useful spells, the choice became melee or die.
Casting useless spells also drives up fatigue, which could be bad if there's an opportunity to cast an especially draining spell later, or if melee is a risk, or if communion is an issue -- useless spellcasting is good for butchering one's own communicants, for instance.