The arguments I've seen are:

a) you would get more competition / lower prices on trade routes from the US to PR. b) foreign flagged vessels could stop in PR on their way to the US, drop off PR goods, and load PR manufactured goods.

It's hard to argue that those possibilities wouldn't have _some_ impact on CoL... but they wouldn't change the bigger reality that PR is an island that doesn't have any natural resources, imports 85% of its food, and is a second class entity within the country it belongs to.