From the RuleBook:
"Piercing-only weapon between 48 and 60 inches in length, must have padding on upper 1/3 of length and pommel and should not be confused with the javelin, which may be thrown. Is considered wooden."
Certain classes use spear as a primary weapon. Certain classes cannot use spears at all. If an Ashigaru is using a polearm, he is in violation of the rules. He must retrieve a spear, forgo the polearm, or sit out the rest of that battle.
Conversely, if a Merchant is using a spear instead of a staff, he is in violation of the rules. He must retrieve a staff, forgo the spear, or sit out the rest of that battle.