I've seen lots of objections to the Cash for Clunkers program based upon the fact that all the program does is shift consumption intertemporally, it doesn't actually create sales that wouldn't have occurred anyway.
But that is not, in and of itself, a valid objection. Shaving the peaks in output and consumption to fill the valleys stabilizes the economy. When the economy is in recession, creating brand new things that wouldn't have existed otherwise to lift the economy back toward full employment is preferred, but generally there aren't enough opportunities along these lines to give the economy the help it needs. In the cases where we cannot create enough new output and consumption to bring the economy back to health, moving consumption from a time when the economy is overheated to a time when it is underperforming helps by bringing both time periods closer to the long-run trend.
Now, Cash for Clunkers is not the the best way to shift consumption from the future to the present, or anywhere close since the intertemporal shifting is generally only for a month or two rather than from good times to bad, so this should not be interpreted as a defense of the program on this basis. But that doesn't mean the idea of intertemporal shifting is inherently flawed.
There seems to be an idea that policy must create something new, that simply rearranging consumption intertemporally is of no value. But there is value in avoiding large cyclical swings in the economy, i.e. value in stability, and when we have the opportunity to shave the peak of housing and other booms - times when the overheating is dangerous as it could result in bubbles, inflation, and other problems - and then use the "shavings" to fill the troughs of recessions, we should do so. [Again, let me stress that I am not supporting the Cash for Clunkers program with this argument, I simply wanted to use that program as a lead in to make a general point about stabilization policy and intertemporal substitution. Also, because I had a very simple model in mind, one, for example, without inventories, I wasn't as careful as I should have been and used the term consumption rather than output.]