I feel bad about writing too many abstruse theory posts. How many angels can dance on the head of a monetary pin?
Here's something more practical. Think of it as a companion to Livio's post about Canadian house prices. Why would we worry if Canadian house prices are overvalued? One important reason (though not the only one) is that if house prices are overvalued that is one reason (again, not the only one) to expect they might fall. And if house prices fall, people with large mortgages might have negative equity, and this might lead to defaults, and problems in financial markets.