well you could Argue that it's even more effective in a hard campaign, as in a medium campaign this could trigger financial aid(if you do it to a not overly rich faction) which more than compensates the decreace of trade. in a hard campaign the AI gets the money anyway so it does not help the AI to blockade his ports, anyway I think most of the time 5000 mnai per turn are not significantly worse than 5100(a rough estimate of profit from trade+scripted help) mnai, unlike the deficit caused by upkeep from blockading ships that even bring the risk of getting sunk by pirates :/