I think the secular decline in various measures of dynamism is a pretty important topic, largely because we haven’t been able to figure out what’s causing it. We’re seeing it not only in worker flows but also job flows, migration, startup rates, etc.
Industry composition effects make the puzzle even bigger–retail and services are typically more volatile than manufacturing, so the larger employment share they’re seeing means we should expect to see HIGHER rates of churning rather than lower (see http://updatedpriors.blogspot.com/2013/02/job-flows-industry-composition-and.html).
One thing that DOES help explain secular declines in gross flows is firm age stuff. Startup rates and employment shares among young firms are on secular decline (see http://updatedpriors.blogspot.com/2012/12/startups-and-great-recession.html); since dynamism typically declines as you go up through the age classes, lower young-firm activity means we should expect lower flows. But it’s not clear that this is a sufficient explanation; and, more importantly, it’s only explanatory in an accounting sense. We don’t know why entry is declining. And this is a secular trend, so common political explanations may not work.
Whether we should be worried really depends on what is causing all of this. After all, churning is costly. If churning is declining for good reasons, we should applaud it. But that may not be the case.