I wonder if there is really much benefit to these ultralight boats. What you save in weight may be sacrificed in other areas.

Unless one is bare boat racing the only real benefit I can see to saving the extra couple of kilos is getting it on the car. Once the boat is in the water there is really no disernable difference and the thin skins, bulkheads, paint jobs, and reduced deck outfitting may compromise convenience and toughness.

Et

Why build ultralight boats? For the same reason why people invest a couple thousand $$ extra on a carbon road bike that weighs only 8kgs instead of 11...
Speaking of the "real benefit": Unlike most people here I don't own a car. I have to carry my kayak to the water if I want to go for a quick paddle. Depending where I want to go distance to the water is between 1,5 and 12km. I use a little bike trailer too sometimes but it's much faster without for the short distance...
Besides, I like the challenge of building lightweight just as much as having to paddle with less effort due to a low weight hull. Btw I am planning to build a hydrofoil kayak in one or maybe two years, so this is also preparation work for that.
Oh and SOF – I just don't like them…