This might seem like a stupid question but let me explain. From what I understand pirates would aim for the ships masts and sails with their cannon but only if the enemy didn't surrender. Chain-Shots were superior at taking down masts and sails and grapeshots were used against the enemy crew. Was there any reason using roundshots instead of Chain-Shots apart from the price which I suppose is higher for the Chain-Shots?
From a ballistic standpoint, the standard round ball was superior in accuracy and in range. The "Chain-Shot" and many of the later antipersonnel rounds would then only be effective at short range and with unreliable accuracy. "A Treatise of Ordinance and Naval Gunnery" by Edward Simpson has an interesting section concerning the historical applications of these different types of projectiles.