I believe the argument for the importance of the process is that
(after sunlight and water), N content is supposed to be the
production bottleneck for most soils. If so, intensified
agriculture without fertilizers wouldn't increase yield, and
mortality reduction would lead to either hunger or a substantial
increase in cultivated land. (according to Smil, the latter would be
possible in the US, but would be unlikely in Egypt, China, bits of
India, etc.)
I don't know how true this is, though. References, anyone?
-Dave