Make it your own with no open source restrictions
Umbraco’s commercial license is included with the Umbraco Pro subscription. This allows you to use umbraco as your own software, including bundling it with legacy software, rebranding the UI or for use in companies with a policy that precludes using open source software.
In other words a commercial license lets you compare, umbraco head-to-head against any commercial closed source product, avoiding any prejudicial sentiments regarding open source software.
Now that could be worth the license cost alone, couldn't it?