[I]ntact mangroves along coasts (parts of Bengal, parts of the Andamans and Nicobars) provide incomparable protection against tidal waves and other disasters--here's a report from the BBC. (And this benefit is in addition to the economic benefits [fish] brought by intact mangroves to local communities.)
From the Asian tsunami to Katrina, the message should be obvious: where they still exist, we must protect our coastal ecosystems; where they do not (as in much of our Gulf coast) we must reconstruct them.
The message is not obvious because we don't have a quantitative measure of the benefits and the costs, including foregone opportunities, of maintaining and reconstructing all coastal ecosystems. One can advocate the preservation or reconstruction of coastal ecosystems - all of them, and regardless of costs - only by assuming that they have effectively infinite benefits.
If protection against disasters has infinite value we should not stop at maintaining and reconstructing coastal ecosystems. We should invest enormous (just a little less of infinite) resources in designing and erecting protections that are even more effective than natural ecosystems.
Actually, protection against disasters has, like everything else, finite value. Moreover, at the margin this value is effectively zero.