Of course the obvious downside to a voluntary savings plan is that it isn't effective at actually creating savings. We know this, because in the United States, we don't have compulsory savings plans, and thus have a voluntary system. So how's that working out? Not well. Approximately half of all Americans save 5% or less of their income[0], which isn't enough for emergency situations. Another study in 2013[1] said 27% had no savings at all. Now the main reason for this is quite simple. A lot people don't make enough to have anything so save.

[0] http://money.cnn.com/2015/03/30/pf/income-saving-habits/ [1] http://money.cnn.com/2013/06/24/pf/emergency-savings/