well, new possibilities for trade might eventually become useful (if we ever get other resources to buy/sell). But, what I think most are saying is that sea routes connecting different parts of the island make it a more dynamic place by adding possibilities for new conflicts or new strategic possibilities within a war.
I wouldn't add sea routes just between cities either, I'd want shorter sea routes between rural regions when it makes sense. I could see sea travel developing between Heen & Eykfar for people that want to avoid traveling through the desert. I can imagine short sea routes hopping across the western coast.
