Building Drupal sites that can share content, themes, modules and even database tables
LSD - Large Scale Drupal
When planning a large-scale Drupal site there are many Drupal architectural decision to make; just because you are planning multiple semi-independent sites does not necessarily imply multiple Drupal installs.
When to multisite
Whilst here are many factors to consider e.g. does the site need to appear to site visitors as one site or many sites, how different the theme and layout will be, what the content editing workflow will be, and whether or not content needs to be shared between sites and sections – as a general rule on whether to use multisite installs or not you can say:
If the sites are similar in functionallity (use same modules or use the same drupal distribution) – there is a strong case
If the functionality is different – there is a weak case
For many multisite architecture is drived by the desire to want your sites to stay as up-to-date as possible regarding drupal module security and when you have different functionalities this is more difficult to do because it takes a little more time since you need to test each site independently.