I could see using this architecture if each portlet is an application unto itself and did need to interact with any other portlets or any common data.

Example: the sample news and weather portlets are in separate war files because they are totally separate applications.

Also, using this type of architecture, it is fairly easy to build a custom portal containing only the desired portlets.

Of course, if there are multiple portlets that provide access to common data, those portlets should all be bundled into a single war file. An example might be a set of portlets that are used by employees to log their work time, view reports on time spent, and so on.