Just look at the ribbon, toolbar, status bar, etc. Not only does this waste screen real estate, it wastes vertical space which is at a premium on widescreen displays. On a widescreen monitor, Pages minimizes this cruft or puts it in palettes and sidebars.
When you've got a document loaded up in a word processor, you're not just trying to read static content; you're working on that content, and the application you're using is a tool for editing and manipulating it. Hiding the chrome in order to show more of the document emphasises the document content as it is in the immediate instant, rather than the potential things you can do with that content, and makes it harder to locate the tools and functions for manipulating the document.
Minimal chrome might make sense in a web browser or PDF reader (at least to the extent that there's a smaller set of functionality necessary for reading documents, but hiding searching/bookmarking/referencing tools is just as bad) but in a word processor, it's much more necessary to have the application's own functional interface exposed.