The only back-up requirements are for locally stored data that does not also reside in the distribution servers. It is easy to install and maintain a large number of machines. Installation of a machine involves a base installation followed by an automated installation of the customized configuration from the software distribution. Upgrades to these machines can occur automatically from then on. Software upgrades are easy and fast.
Machines are much more secure and reliable.
Security patches can be quickly distributed because of number 3 above. As well, reliability problems can also be addressed quickly because a machine's configuration is in a known state that can be compared to other, working, machines. All machines contain a consistent set of files. Since there is a high degree of control over which files appear on a given machine, it is much easier to debug problems. Also, such machines will have more consistent configurations, and thus more consistent behavior.