Because when it works, it works. But when things go funny, it can take a lot of time to fix.

I work for a company of about 150. We have two people who manage the Exchange server(s). But they don't work on it full time. In fact, much of the work can be done by one person - it's just that one of them does the Windows servers generally and the other does desktop support. So it's natural for the Exchange administration to fall to both of them.