[hider=Weapon and Armor Proficiency][quote=Complete Adventurer p13]
[b]Weapon and Armor Proficiency:[/b] Spellthieves are proficient with all simple weapons and with light armor but not with shields. Because the somatic components required for spellthief spells are simple, a spellthief can cast spellthief spells while wearing light armor without incurring the normal arcane spell failure chance. However, a spellthief wearing medium or heavy armor or using a shield incurs a chance of arcane spell failure (see page 123 of the Player's Handbook) if the spell in question has a somatic component (most do). A multiclass spellthief still incurs the normal arcane spell failure chance for arcane spells received from other classes, including those stolen from arcane casters (see the steal spell ability, below).
[/quote][/hider]
Interestingly enough, they have less proficiencies than the rogue class. Which also brings to light the fact I never thought of, let alone have seen, a rogue using a quarterstaff, longspear or similar two-handed simple weapon. You could even do the same with any weapon that is at least one-handed but not light to gain the one and one-half times bonus from Strength.

I guess I will have to be creative with this knowledge in the future, [@ELGainsborough].