Personal Savings rate of a Nation - How much is enough?

There was once again another op-ed in the New York times last week bemoaning the US's low savings rate and how other developed nations in Europe are saving a lot more.  The author pointed to a data that savings rates have recently fallen below 4 percent.  So I wondered where that info would have come and stumbled upon the data on the Bureau of economic analysis website (http://www.bea.gov/iTable/iTable.cfm?ReqID=9&step=1, abbreviated data attached below).
http://www.nytimes.com/2011/11/25/opinion/why-we-spend-why-they-save.html

[Billions of dollars] Seasonally adjusted at annual rates
Bureau of Economic Analysis
Last Revised on: November 22, 2011 - Next Release Date December 22, 2011
Line		2011
		I	II	III
27	Disposable personal income	11,481	11,559	11,565
28	Less: Personal outlays	10,902	11,003	11,131
29	Personal consumption expenditures	10,572	10,676	10,799
30	Personal interest payments /3/	160	156	160
31	Personal current transfer payments	170	171	172
32	To government	97	97	98
33	To the rest of the world (net)	74	74	74
34	Equals: Personal saving	579	557	435
35	Personal saving as a percentage of disposable personal income	5.0%	4.8%	3.8%

 This is aggregate savings rate and doesn't tell us anything about the more important inter-generational savings rate.  Let's make a bullet list of why we need to save in general:

• To tide over any short-term job loss.  My rule of thumb would be 6 months of earnings.  You can't save for long-term unemployment - long-term unemployment is due to structural problems in the society and a savings rate boost won't cure it.
• To pay for a big expense in the future that can be forseen today - like kid's college education.  Again rule of thumb would be to have atleast 50% of each kid's college expense saved by the time they enter college.  
• To pay for the down-payment on a home - the 0% down-payment days are gone, but one can still get decent mortgage rates with a 10% down.  Again a savings goal could be to have the downpayment saved in say 5 years time.   
• To tide over some unexpected expenses that happen every now and then.  This could be for a hospital stay, a car repair, a home repair etc.  Rule of thumb would be like one month of pay.  You are not going to save for a Cancer care or a heart surgery - one has to have insurance coverage to pay most of those kind of high dollar expenses.   
• Save for Retirement - this is the biggie.  This topic is worth several blog articles.  One has to be reasonable on it - everybody can't retire at 55, can't retire in the Caribbean islands, can't expect a lavish lifestyle.  I see a lot of bank websites that show huge amounts of retirement money needed on their retirement calculators.   

We shouldn't expect retirees to save on the social security or the dividend checks they receive every month.  We can't expect every citizen to save and so the society needs to have safety nets to protect the basic necessities of the population that doesn't save.  People who can't save shouldn't expect to buy a home but keep renting.  There are different savings needs and so different savings rates needed for the different age groups.  There is no point bemoaning the low aggregate savings rate of a nation.  A net export nation can sustain a high savings rate but every nation can't be a net exporter (mathematically impossible).  A high aggregate savings rate in a balanced economy (that doesn't have huge trade surplus or deficit) points to large inefficiencies in exchange of goods and services.  A nation can have an aggregate savings rate close to zero and still be a vibrant society as long as there are different inter-generational savings.