If you don't install the promotions plugin, your console log will sometimes be blank, and your the build.xml for each build will not be saved, resulting in lost data.

In the logs, you will see this error:

Jul 10, 2012 4:08:04 PM hudson.model.Executor run
SEVERE: Executor threw an exception
java.lang.NoClassDefFoundError: hudson/plugins/promoted_builds/JobPropertyImpl
at org.jenkins.plugins.appaloosa.AppaloosaRunListener.onCompleted(AppaloosaRunListener.java:47)
at org.jenkins.plugins.appaloosa.AppaloosaRunListener.onCompleted(AppaloosaRunListener.java:42)
at hudson.model.listeners.RunListener.fireCompleted(RunListener.java:178)
at hudson.model.Run.run(Run.java:1458)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:238)

Rating this as critical since it results in data loss.