Let's say you have a web application Maven project (type=war) that depends on another web application Maven project (dependency of type=war).
The goals jetty:run-war and jetty:run-exploded works fine (since the war is overlayed with any dependant web applications), but the goal jetty:run does not take dependent web application(s) into consideration. The result is that either you work in rapid development mode (jetty:run) with a partial web application [without artifacts from dependant web applications)] or in "slow" development mode (jetty:run-war/jetty:run-exploded) with a complete web application [with artifacts from dependant web applications]!
You can achieve rapid development mode with dependant web applications by preceding jetty:run with war:inplace, but that will place artifacts from dependant web applications into the source tree of your web application - which we don't want, for obvious reasons.
Here's a link related to the subject: