Scenario: user installs a new agent plug-in that defines a new server type and wants to manually import a resource of that type. Problem: when clicking on the "Import" button in the inventory view, the new server type doesn't appear in the pop-up menu. To refresh that list, it is necessary to restart the RHQ server.
I noticed that similar issues exist also in other areas. E.g. after a plug-in upgrade that adds new properties to a resource configuration, I had troubles making RHQ display the new properties (note I checked that they were present in the DB). Probably configuration definitions are cached somewhere and that cache should be flushed after a plug-in upgrade.
The issue you are seeing here is caching of (meta)data in the GWT app and thus in the browser. "Just" reloading the UI should be enough -- when you restart the server, you normally get "disconnected" from it and have to refresh and re-login. Does that solve your issue? I know this is not perfect, but luckily one does not re-deploy plugins that often.