I often get asked "how many document libraries and sites will we need?" in SharePoint, followed by "how will we know whether we should use more doc-libs in a site or just throw it all in there?" and "where should content be stored? we need to show it on the intranet home page, but it is really edited and owned by HR in region Gokk". Well, SharePoint is like a chest of drawers.
To answer such questions, you need to know how to classify and structure your content; ideally you should have an Information Architecture for all your different kinds of data. If you are like most others, you don't. This is where the "chest of drawers" analogy might help you reason about your content.
Whether you need many doc-libs or subsites or not depends on your IA policies for information management. Still don't have an IA? Think of a chest of drawers for you clothes: it makes it easier to manage different types of clothes in different ways at different schedules, by e.g. separating t-shirts from trousers. Maybe even handle different kinds of t-shirts differently, such as your precious Maiden t-shirts. It also allows for delegating a few drawers to be managed by your wife; maybe you even want to have some locked drawers with more privacy :)
Your content are the clothes, the drawers are doc-libs or even subsites; and depending on the variety of clothes you have, you might need quite a sophisticated chest of drawers. Throw in all your other stuff, and you might need a bigger closet or a garage!
Think of a news paper with a front page and then multiple sections, such as domestic, foreign, sports, economics, etc. The front page and section front pages are used to show the (elsewhere) stored content to readers, helping them quicly browse the content at wellknow locations in the paper. The shown stories are typically rollup content stored elsewhere, typically where maintained, close to the content editors.
So a paper is built from dispersed storage of content that can be rolled up and targeted to readers multiple places. The home page and section pages rollup content "teasers" and allows the user to browse the content stored elsewhere in the paper and decide whether to explore it further.
The front page is the home page of your SharePoint site, the sections are subsites and the section pages are the subsite welcome pages in SharePoint parlance. As for the drawers, there might be different management policies and different people handling the different sections, and this helps you decide when subsites are needed. The rollup, or cross publishing if you like, is achieved using the content by query web-part or search-driven content based on content types, tagging and metadata.
Controlled and secure management of content according to different policies and schedules is much simpler when using subsites as compared to throwing it all into one site. Store the content close to the producers, show it everywhere the users expect to find it - and also where *you* want them to discover and explore knowledge new to them.