It seems to me that those Sys Admins who do evangelise monitoring are on a different level to others. This is my own experience talking, and not based on any quantitative evidence.

For instance my current employer had no monitoring at all when I joined and was content with this. Instead they relied upon the customers contacting them when there was a problem, which in turn didn't make they appear very bright when replying "Oh, really? I'll have to test that and get back to you...". A big benefit of monitoring prevents this from happening.

For me personally though, I need monitoring because I view my network as my baby and I want to know everything that goes on - like an overprotective parent! But in all seriousness, it's just common sense to use monitoring with the increase in the size of networks that a Sys Admin is responsible for (I'm far too young to know if this is true but I'm guessing the ratio of Sys Admins to servers/network devices/customers has increased in the past few years).