While you are correct that in a system without greed communism could surely benefit the economy, everyone chips into different types of jobs, some harder, some easier, however, these jobs are still equally as important. On an individual level, it would be unfair, fair in the definition of people getting what they deserve, for me to be receiving the same as someone who doesn't work as hard. This could also pose the problem that most people will shoot for lower effort jobs with the same pay, creating an imbalance of jobs that are completed and jobs that aren't. It's also a very big point of demotivation of some. In the perfect society, I do admit, it could work, but that would look much more like if people weren't people and didn't feel envy or jealousy, not just greed, which is something that really strips the humanity from a person.