The Java EE jars in the javax.* groups in the Maven Central repository are the ones provided by Sun. What if you want to use another vendor's specification and implementation jars?
Unfortunately, Maven doesn't understand "is an implementation of", so if you declare a dependency on one of Geronimo's Servlet 2.5 API jars, you may also end up with Sun's servlet-api-2.5.jar on your classpath as well.
The same applies if you host an internal repository and want to use IBM's or BEA's Java EE implementations, which aren't available in the public repository.