Playing PVE I've never used harpoons, it's a PVP thing, but seeing it writen down made me realise that some one new and hunting whales for the first time would assume "harpoons" would be the ammo to use.
At the start there was only balista bolts, harpoons came along later for PVP'ers to grapple each others ships. They realy ought to change the name to avoid confusion.