It is currently possible to specify a archetype catalog through a file: or http: URL (via the archetypeCatalog param). This adds the possibility to go outside of the configured Maven infrastructure which causes problems and unpredicted side-effects.
I strongly argue that the plugin should work with the Maven settings and infrastructure in place and not allow for going outside of this. For example, in a setup where there is a Maven repo manager in place it should always be used and the user should not be allowed to side-step it through the support of the plugin. If an additional source should be used, it should be setup properly in the Maven configuration/infrastructure.
I also believe the current support is a legacy from the old days when the catalog at central was not complete, but one had to used additional catalogs in the Internet.