The test portlet provided will store messages in its 'own' session and preferences. By adding multiple instances of the portlet to the same and different portal pages, you can see whether these 'private' messages leak into other instances of the portlet.
While testing, you will be seeing whether portlet instance sessions are treated independently during a single browsing session. Another thing to check is whether user preferences for a portlet (those which are saved and persist across different browser sessions) are also kept separate for portlet instances - in most cases this should match the session behaviour, but it's not guaranteed.
Explanation
This issue will affect you if you plan to be implementing a portlet which can appear multiple times on a portal, configured in different ways using preferences.
For example, you might want to add a Weather portlet twice to the same page, and configure it to show the weather for different locations. Or you might want to add it to more than one page, again showing different locations.
Each portlet is registered by adding its entry in the portlet.xml. However, if a portlet only appears once in the portlet.xml, should the user be able to add multiple instances of that portlet to a page, and maintain different states and saved parameters for each instance? JSR 168 is unclear on this, so different portals have ended up implementing this in different ways.
Some portals require a strict mapping of portlet.xml entries to portlet instances: to allow multiple Weather portlets on a page, you would have to register the portlet multiple times (with many nearly-identical entries in the portlet.xml). This is the behaviour you will see in Apache Pluto.
Other portals automatically treat each user-added portlet instance separately. Oracle Portal and Jetspeed behave this way.
There are yet more possibilities: for example, at the time of my testing, Liferay did not permit multiple instances of the same portlet on one page, but instances on different pages were allowed. (This may have changed in later versions.)