This was so much easier to do back about 40 years ago , it was the way we kept freewheels going . Today it not so easy , because single cogs are not sold to do so . The best you can do is get another cassette with the cogs you want or need that are not wore as other are . As with most people yes the middle ones are the use one . Other than that yes I do agree with you it is a waste . If anybody have any for sale these guy do , Loose Screws Bicycle Small Parts Hard to find bicycle parts, made easy! For both the classic and modern cyclist. .