Ask yourself: How do your users react when you announce (or plead for) a downtime window to accomplish an upgrade or to perform maintenance? Not well, I'd imagine.
Years ago, scheduled downtime was a common occurrence in all but the very largest IT shops, but today, few businesses let you get away with a solid downtime window without an act of Congress. Even some shops without obvious 24/7 requirements -- like three-shift manufacturing plants or hospitals with emergency rooms -- have a hard time denying their user base access to data even in the wee hours of the night.
The reasons for this are many, but they boil down to a voracious dependence on IT systems for day-to-day business -- and massively improved disaster avoidance brought about in large part by the advent of server virtualization. Businesses are addicted to data; technology has improved to the point that we in IT can readily feed that addiction.