Why should we even consider Social Safety Nets?
Well, one reason would be to maximize production of our society. If you say that every person represents a substantial initial investment of resources, it might be worthwhile to prevent people from sinking so low economically that it becomes near impossible to recover and become self sufficient again.
One reason against Social Safety Nets would be causing a drain on productivity. If the people who sink low enough economically to trigger a Social Safety Net are then satisfied to live at that level, they will never become self sufficient again, and be a drain on those who are.
To me, it seems the best course would be to find the middle ground between these two phenomena. I would suggest that a Social Safety Net be set high enough so most people would be able to recover, but low enough that most people would be motivated to recover. I suppose what I am really suggesting is a Social Safety Trampoline (one that propels you back to prosperity when you hit it). No one level will work for everyone, as some will have a very low tolerance for poverty, and others a very high tolerance for poverty. What we want to do is set it so the (super?) majority are motivated to return to self sufficiency. I'm not sure what level this is, exactly, and it would be helpful to figure this out before setting the system to work. Also, I think it could be slowly raised over a long time, as people will gradually come to associate higher and higher incomes with insufficient amount of income in comparison to those they have contact with. If eventually the level where you are barely self sufficient becomes intolerable to the vast majority, then the safety net would no longer need to rise, and as an act of mercy those few who are still satisfied to remain below that level could be funded as they would be a negligible drain on the economy.
Now, the whole underpinning of the idea of the Social Safety Net is the concept that it is better to live in a society where everyone is able to have their basic needs provided (preferably by themselves), then to live in a society where there are those who do not have those needs provided for, regardless of your own prosperity. If you disagree with this proposition, then there is no need for a Social Safety Net (or even a trampoline).