Nick Rowe--professor of economics at Carleton University in Ottawa, ON, and one of the authors of the Worthwhile Canadian Initiative blog-- argues that there is value in bubbles.  A lot of value.  At least in economies where the rate of growth out-paces the "natural rate of interest."  

In this diagram, the Y axis is the real rate of interest, while the X axis is "the real value of the stock of assets."

Rowe writes:

Economists normally define desired savings as a flow demand for assets. Here I want to think of it as astock demand for assets. Think of an overlapping generations model in which people desire to accumulate a certain value of a stock of assets for their retirement. And for emergencies and other lean years. I have drawn the savings curve as upward-sloping, so people's desired value of their stock of assets is an increasing function of the rate of interest. But that is not essential to my argument.

Economists also normally define desired investment as a flow. Here I want to think of it as a stock supply of real assets. It's the fundamental value of the stock of capital plus land, where "fundamental value" means the present value of the flow of earnings. The investment curve slopes down for two reasons: because a lower interest rate means a higher present value of a given flow of earnings; and because more investments become profitable at a lower rate of interest.

Read Rowe's full explanation here.  (H/t Mark Thoma, Economist's View)