What about retention/archiving policies? A mail server should never run out of space.
I'm not saying it should but again, that's down to the network / sys admin coupled with the business rules.
But Delete nothing, Isn't always practical for small enterprise. That's why we have CMS systems / bespoke databases. You dont have to be in people's inbox's to see correspondence for customers, suppliers etc.
Personally I only have items which are relevant to what projects I'm working on at any given time in my inbox. But that's just me