a moral argument
I hope that society has gotten to the point where we really do feel like there is a moral obligation to take care of those who have fallen through the cracks somehow. Given the way that the economy is shifting - less loyalty from employers to employees, reduced wages (check out the details on Rick Santorum's (R-Sen, PA) bill that _supposedly_ raises the minimum wage but really does exactly the opposite), outsourcing of jobs to foreign countries, and rising cost of health care, among other things - Social Security is the least that this government can do for its people. Instead, the Republicans currently in power keep slashing taxes for the rich, cutting basic social programs, and making things harder in general for the average person to get by. Why do we keep voting these bastards in?
The real point, though, is that when you set aside all the practical matters of debt and transition costs, this is an ideological debate -- or to put it less antiseptically, a debate over different sets of values.
The idea behind private accounts is that people should rely on themselves alone and bear the consequences of their successes and their failures and random chance on their own shoulders. If things don't pan out for you in retirement, that's something to take up with your children.
The concept behind Social Security is fundamentally different. The first premise is that if you put in a lifetime's work there is simply a level of destitution below which society will not let you fall. Maybe you made so little during your working years that there wasn't enough to save. Or maybe you just didn't plan ahead well enough. Or maybe you suffered some misfortune. Whatever. If you worked you won't be destitute when you retire. People who made big bucks through their lives don't get a particularly good 'deal' from Social Security, if you insist on seeing it in investment terms. But that's a distorting prism, sort of like thinking you got a rotten deal on your medical insurance if you never have a catastrophic illness.
I like to think of this as the moral equality of work. In our society, we allow the market to assign all manner of different cash values to different sorts of work or even the same sorts of work under different circumstances. And by and large, within some very small limitations like the minimum wage or certain non-discrimination laws, most of us think this is how it should be. I certainly do. (In this sense, I think collective bargaining amounts to another competitive arrangement within a market economy -- though doctrinaire free market folks have always seen it in contrary terms.)
But the cash value of work isn't the same as its moral value. And if you look at the values imbedded in all those Social Security actuarial tables, you see this principle: whether you were a janitor or a fast-food worker or a doctor or a tycoon, if you worked during your working years you shouldn't be left destitute when your working years are over (retirement) or when, through no fault of your own, you can't work anymore (disability). No matter what. The common denominator is a life of work -- skilled or unskilled, impressive or unimpressive, remembered or forgotten. It doesn't matter.