As I consider this further, I think the current still does have some impact on different points of sail - consider reaching when the current and the wind are in the same direction and you are moving perpendicular to them both...the water movement does subtract from the overall wind here because the boat is having to directly compensate for the additional slip to leeward that subtracts from (or push to windward that adds to) the wind. On this point of sail, if the boat doesn't have any friction in the water, they're just sliding down the course.