A little something from the Tax Foundation:

Homeownership has come to be viewed as an integral part of The American Dream, and tax policy is just one way that politicians have sought to promote it. The mortgage interest deduction is the second most expensive tax subsidy, second only to the tax exclusion for employer-provided health insurance.
From the Tax Foundation. Dated 2 December 2009

Okay. But then they really should say it's the second most expensive tax subsidy of the individual income tax. As opposed to the corporate or business income tax, where almost every dollar spent is tax deductible. Advertising, for example. Think there are too many ads on TV? Get your Congressman to sponsor a law that eliminates the tax deduction for business advertising expenses.

The difference between the individual and business tax systems is universally assumed to be proper. I think it leads to tremendous distortions in spending. I think that kind of thinking ultimately leads to Supreme Court decisions that say things like corporations are people, too, and the recent Citizens United v. FEC decision that spending unlimited amounts of corporate money to influence political campaigns is simply free speech.

Ironically, one category of business spending that is not legally deductible is the political and lobbying expenses category.