If you have a Arcane Warrior who became tranquil, it stands to reason that while he can no longer use magic, he would still be plenty handy with a sword...if there's plenty of tranquil being trained for administrative stuffs, organising, cleaning, etc, why not have one excel at what he is still slightly better in? Heck, he could be a useful sparring partner at times (or a completely logical and emotionless assassin if you think about it). Now, if you put a logical motivation up in the air, as in...stopping the blight will need more warriors to fight for the cause, it wouldn't be entirely crazy for a tranquil to decide that the logical thing to do is to aid the group, even if the situations and combat to come are uncomfortable.

Of course, I could just hire him to procure you specific items/weapons/armour.

The Tranquil can leave anytime they want, it stands to reason that at least one or two would leave...emotions or not, passions or not.