Yeah Flynn that must be firaxis's intended purpose for them since they serve no other.
And whats more annoying is that these have a high variable to be chosen to autobuild. I have a game where I killed off the other enemies except one, and was at war. But every damn time I finished building something a privateer would be selected next!! (Only 2 players, its pretty obvious who owns what units, hint, if its not yours, then its his

)
So of course I built quite a few of these by mere accident, since by that time I had massive production, some would come out in one or so turns. I read in the manual that the governer remembers what you have built when selecting new things, if thats the case then privateers are the only thing that will ever be selected there, since most buildings I only build once
Whats the defensive bonus on water?
I'm fairly dissappionted in the naval combat, I need at least 5 galleys to take out an enemy galley, and ironclads etc, forget about it, I try to ignore them. Bombarding seems the only worthwhile way to kill naval vessels.