I agree that something unusual is going on. Humans, unlike any other species I'm aware of, are voluntarily restricting our own population growth. But I don't know why you say that there's "no reason" to believe that this strange behavior might benefit us. Surely you can think of at least one reason? After all, all those other species that don't voluntarily limit their own reproduction eventually see their populations crash, or level off in the face of fierce competition over resources, when they meet or exceed their environment's carrying capacity. The laws of physics as we currently understand them dictate that exponential growth cannot continue forever.

I'm not saying that there are no foreseeable downsides to population leveling off. And I'm not saying that there's no risk of unforeseeable consequences of the social changes underlying this demographic shift. But I am saying that (amid all the pros and cons) there is one obvious, important reason why human population leveling off might be a good thing. The downsides are neither so obvious nor so potentially dramatic. To illustrate this, lets look at Last's (awful) WSJ article quoted in the Marginal Revolutions post.

Last does his best to paint declining fertility as a nightmare scenario. But the data he presents simply don't support his tone. For example:

Low-fertility societies don't innovate because their incentives for consumption tilt overwhelmingly toward health care. They don't invest aggressively because, with the average age skewing higher, capital shifts to preserving and extending life and then begins drawing down.

In other words, low-fertility societies do have an incentive to innovate - in medicine and life extension. And not just for the benefit of the old - they also have an incentive to keep the young healthy and productive as long as possible, to maintain their shrinking workforce (which may go some ways toward explaining Japan's excellent school nutrition program, and low, declining childhood obesity rates). They also have an incentive to develop automation to replace aging workers, which I know is a major reason that Japan is a leader in robotics. Let's take a closer look at Japan:

From 1950 to 1973, Japan's total-factor productivity—a good measure of economic dynamism—increased by an average of 5.4% per year. From 1990 to 2006, it increased by just 0.63% per year. Since 1991, Japan's rate of GDP growth has exceeded 2.5% in only four years; its annual rate of growth has averaged 1.03%.

Wait, did he just admit that Japan's economy is still growing? Yep, both GDP and GDP/capita have continued to grow, albeit more slowly, since the 1990s. Let that sink in a moment. The Japanese are, on average, working less than they used to. They're older and more likely to be retired. And yet they still get to enjoy having more stuff. (Largely thanks to innovations in automation driven, in part, by aging demographics.) And thanks to medical innovations, driven in part by aging demographics, they will continue enjoying that stuff longer than any generation before. So where's the grim cautionary tale? Last has none, just this:

At the current fertility rate, by 2100 Japan's population will be less than half what it is now.

Which would still be more than it was in 1900. So, where's the problem? Why is it preferable to keep taxing the earth's resources with more and more people with no foreseeable prospects at space colonization? On overpopulation, Last says,

First, global population growth is slowing to a halt and will begin to shrink within 60 years.

This is just unforgivably bad logic: 'Overpopulation isn't a problem, because population is leveling off, because fertility is declining. Therefore we must act immediately to put a stop to declining fertility!' If we ever do face a shrinking population, I'd rather deal with it by increasing healthy lifespans than by increasing birthrates.