Wait, why are motorcycles important? They're inherently less efficient, netting out roughly the same environmental impact --- assuming modern motorcycles! --- but without the ability to carry passengers or significant amounts of cargo.

There are tens of thousands of donor organ recipients who wouldn't be alive today if it weren't for motorcycles.

Maybe in the west where motorcycles are more of a leisure transport than a mode of commute, but in developing countries bikes with small engines in the ball park of 100 cc (the honda cub for example) are definitely some of the most efficient modes of transportation. In India i could get a bike that could comfortably get around 60kmpl in normal driving conditions ( 3 bikes with 6 passengers total would be about the same as a prius at about 1/10th the cost). And in a congested city, no other mode of transport beats it (unless the city has a very well connected metro rail transportation)