cont...and receive back a "living wage" sort of stipend.
This sound GREAT for the lazy and uneducated, since they have to contribute nothing ("...according to his ability...") and get back enough to cover the basic necessities.
Trouble is, all those "fair share" societies that have tried this among the proletariat have discovered that a "fair share" is not enough incentive to keep smart and hardworking folks at the grindstone. Just as in a slave-based economy, the folks getting that "fair share" (or the slaves) will work just enough to prevent punishment and receive the stipend. Productivity drops precipitously in a "fair share" environment.
Suddenly, the pie that the "fair share" is cut from becomes smaller, as does the share.