I backed a quilt with lawn a couple years ago, and I love it! The lawn is very soft and smooth. I've also made a couple summer shirts out of it.
Cotton lawn is woven from finer thread than quilting cotton, but the threads are usually closer together, so it's not open like a gauze. I wouldn't use it in a utility quilt because it will probably wear faster than quilting cotton, but for a couch throw it's very nice. I like it for backing because it is generally 55ish inches wide, so you can make a wider throw without piecing the back.