maven provides scope (test, runtime etc.) to filter certain dependencies.
never the less sometimes it is quite helpful to have a another configuration option, where you decide what to include transitively or what kind of dependency-set is relevant to your current project. using <exclusions> configuration to accomplish this is often very verbose, non-standard and is difficult to read (it is often not obvious what is meant with all these exclusions).
as an example ivy includes such configuration meta-info approach (see http://ant.apache.org/ivy/m2comparison.html). this feature would be nice because when building releases one is more flexible.