Do we need Fannie and Freddie?

Two recent articles coming out quite differently on this question.  In a WSJ op-ed the American Enterprise Institute's Peter Wallison argues that most other countries get along fine without any implicit federal guarantee of repayment.  In a NYT op-ed journalist Bethany McLean raises concerns that banks would make significant changes in mortgages if government guarantees were to vanish, such as ending 30-year and fixed-rate mortgages. 

In simple economic terms, McLean essentially argues that interest rate subsidies for housing are socially worthwhile whereas Wallison is unconvinced.