If the workers work less there is less supply. Less supply means more demand, which cant be met. Prices go up and up and up and eventually you got an economic disaster based around the price rise of basic human needs. Look at venzuala, although its completely from your idea of anarcho-communism (they, are state socialist), they have this exact problem. The population became lazy and this had a similiar effect as to when you only produce what you need. What you need isnt much, but it sure helps with quality of life when you get what you want.

I also realize that you may disregard supply demand in your communist theory because commies saught to evaporate the economic cycle.