I read once in one of Hurley's books (I believe) that the sea raiders had smaller spears which could be thrown by the handfull when approaching another vessel in order to keep the enemy crew disorganised until boarding .
Not so sure about the usefulness of a full sized spear on a fighting Prau .
If you look closely at the hafts of the harpoons shown in the pictures you'll notice they are quite long (12 foot plus) and thick .