In my limited experience the harpoon is not only good for snagging an enemy but also as a funny means of escape. with a good head of steam you can use it to slingshot past your target if you don't wish to remain close after having, say, given the business with your goldie's heavy carronade.
one thing I am def curious to see is how the tethering effect stacks, if at all, with more than one harpoon.
also, the harpoon description suggests it does hull damage primarily while the stats say balloon. is it just one or both?