Let me give it a try (where's that darn FC hat?):

If you are a fiscal conservative who thinks that the progressive tax code is far too skewed, almost any policy which reduces the tax burden at the high end is better than none at all. If you are a pro-economic growth conservative, a sustainable policy which results in higher real estate valuations would be good. That is what the MID presently does. Removing the MID would cause short-term economic dislocation, and misallocation of investments, and a fiscal conservative would prefer a less volatile investment environment. A fiscal conservative (and a savvy liberal) normally just accepts the rules as they exist, and adjusts their investments accordingly to take advantage of policies, rather than bellyache.

BTW, JSA is right that home ownership, like marriage, often leads to an entire set of economically "virtuous" behaviors which help teh economy and society as a whole (saving every month, holding a job for decades, staying married, spending on "stuff", etc.). That isn't a case for the MID, but it is a general argument in favor of home ownership.