You will not only find high aspect ratio wings on sailplanes but on planes designed for duration and long distance flight. The 10 foot span aerosonde that flew from Nova Scotia to Scotland (about 1700 miles) nonstop had an aspect ratio of about 14. Bert Rutan's plane that flew nonstop around the world without refueling had high aspect ratio wings. The reason is that high aspect ratio reduces induced drag due to the vortices at the wing tips. At medium and low speeds, induced drag is more than half the total drag of the airplane. Anything that reduces drag reduces the thrust needed and increases fuel economy, range and duration.