i don't really see the point of island settlements, namely Cagliari and Ajaccio. or Sicily. Why Bother taking them over? If the enemy has them, better to leave them be if you've got naval superiority, effectively stranding them.
Sure, settlements like Nicosia might be fine as a launching pad for raids and invasions but by that time you should already have other shore based settlements rendering them moot.
Does anyone actually find island settlements to be useful? Are any of them particularly good money makers on the order of Venice or Constantinople?