There's a related issue
MNG-2796 (http://jira.codehaus.org/browse/MNG-2796) involving some issue with using one pom ("server" parent pom) to depend on another pom ("container" parent pom). The desired effect is that any children modules of the container parent pom cannot depend on any children modules of the server parent pom. Having the server parent pom depend on the container parent pom is one way to ensure this cannot happen, but maybe considered a hack – and I suppose it is when you consider that this only works if you control the poms of the deps that are "off limits".
Brett has suggested adding explicit support for this so that groups of deps or individual deps could be marked as disallowed. That feature would definitely work as an alternate ensuring that nothing in the container group depends on anything in the server group.