Two simple rules of thumb follow:

1. If one of the roughly ten Portal-only features are required, use a
SharePoint Portal Server Area otherwise use a Windows SharePoint Services
Site.

2. Only if the information to be contained is relatively static and
long-lived, primarily read-only and not in flux (collaboration should
typically be done in a Windows SharePoint Services Site), and enterprise  in
nature should it be housed in a SharePoint Portal Server Area. Examples
include: company news (obviously, potentially targeted), HR guidelines,
policies, and procedures, vacation request forms, annual corporate  reports,
press releases, etc.

I've found other factors also come into play:

3. What type of "presence" a group wants to have on the intranet. Some want a "portal" and need to be convinced they either do or don't.

People that design and contribute to a SharePoint Portal Server's Areas
should be gatekeepers for enterprise-wide only data. They should only be a
handful of them even for the largest organizations.