I find them best for building. Farmer-Runners seem to get "caught" on buildings a lot and you lose food. Builder-Runners clear the beach and build a hut faster. I also like to give them the healing skill because later when you need them they make the quickest doctors.
Even though it may seem like food is lost, it really isn't; the food only gets deducted from the harvest ground when it goes into the food bin. Plus, there are no buildings between the ocean and the food bin for a runner to get caught up on.
On the flip side, I've had runners trying to build things, get caught on a building, and lose their building materials. I personally prefer putting an early-game runner on building since the lumberyard and the rock quarry are on opposite sides of the village, and late-game or end-game runners get to fish to their heart's content.