While housing prices are important to the economy, the level of turnover in used houses is not. Home equity is, or at least was, the most important store of wealth for the vast majority of families. Houses are generally a very leveraged asset, much more so than stocks. Using your full margin in the stock market still means you are putting 50% down. In housing, putting 20% down is considered conservative, and during the bubble was considered hopelessly old fashioned.
As a result, as housing prices declined, wealth declined by a lot more. For the most part, we are not talking vast fortunes here, but rather the sort of wealth that was going to finance kids' college educations and a comfortable retirement.
The decline in housing wealth is a very big reason why retail sales have been so weak. With everyone trying to save, aggregate demand from the private sector is way down. If customers are not going to spend and buy products, employers have no reason to invest to expand capacity. They have no reason to hire more workers.
Underwater Mortgages Lead to Foreclosures
Also, as housing prices fell, millions of homeowners found themselves owing more on their houses than the houses were worth. That greatly increases the risk of foreclosure.
If the house is worth more than the mortgage, the rate of foreclosure should be zero. Regardless of how bad your cash flow situation is -- due to job loss, divorce or health problems for example -- you would always be better off selling the house and getting something, even if it is less than you paid for the house, than letting the bank take it and get nothing.
By propping up the price of houses, the tax credit did help slow the increase in the rate of foreclosures. Still, 23% of all houses with mortgages are worth less than the value of the mortgage today. Another five percent or so are worth less than five percent more than the value of the mortgage. If prices start to fall again, those folks well be pushed underwater as well.
On the other hand, it is not obvious that propping up the prices of an asset class is really something that the government should be doing. After all, it is hurting those who don't have homes and would like to buy one.
Support for housing goes far beyond just the tax credit. The biggest single support is the deductibility of mortgage interest from taxes. Since homeowners are generally wealthier and have higher incomes than those that rent, this is a case of the lower middle class subsidizing the upper middle class. Also, even if they are homeowners, people with lower incomes are more likely to take the standard deduction rather than itemize their taxes. The mortgage interest deduction only applies if you itemize.