A solid roof rack is better for hauling boatbuilding lumber than a truck bed. With the rack, you can haul those 18' cedar and redwood planks. I even hauled a 21' construction tube filled with pre-milled strips on a roof rack. It sagged a bit at the ends, but made it home safely. I don't think a trailer would have worked very well.

Once you've built your boat, the roof rack gives you some added security for hauling it. On a trailer, the boat is easier to access by yak-jackers. It would also be at a height where a rear-end collision would trash all your hard work. A roof rack moves it out of the way.

My wife and I each have Thule racks on our cars (Ford Explorer, Land Rover Defender) and we can switch the saddles from one vehicle to the other. At least, that was the original plan. We've done that many times, but it turns out that removing/installing/aligning/positioning the saddles takes some time. When I finish my Guillemot, we'll be up to 4 boats, and we'll have to take both cars to haul them all anyway. The saddles aren't cheap (about $80 a set), but they really grip well.