Scala-xml module is on the bootclasspath of the compiler because it's needed by scaladoc. However, Scala compiler will pick up all jars from bootclasspath (unless -nobootclasspath is passed). That's the reason Scala 2.11 compiles code containing xml literals even if we do not put scala-xml on the classpath explicitly:

$ cat XMLHelloWorld.scala 
object XMLHelloWorld {
  def main(args: Array[String]): Unit = {
    val xml = <hello>world</hello>
    println(xml)
  }
}
$ ./build/pack/bin/scalac -d sandbox/ XMLHelloWorld.scala

For command line scalac that's the behavior we probably want to have. However, we get the same behavior in Maven. Consider project with the following pom.xml:

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <groupId>sample</groupId>
  <artifactId>scala-module-dependency-sample</artifactId>
  <version>1.0-SNAPSHOT</version>
  <dependencies>
    <dependency>
      <groupId>org.scala-lang</groupId>
      <artifactId>scala-library</artifactId>
      <version>2.11.0-RC1</version>
    </dependency>
  </dependencies>
  <build>
    <sourceDirectory>src/main/scala</sourceDirectory>
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-compiler-plugin</artifactId>
      </plugin>
      <plugin>
        <groupId>net.alchim31.maven</groupId>
        <artifactId>scala-maven-plugin</artifactId>
        <version>3.1.6</version>
      </plugin>
    </plugins>
  </build>
</project>

If you try to compile (mvn scala:compile) the same code as above with Maven Scala plugin, it will succeed. However, there's no dependency on xml declared so if you try to run your code you get:

mvn scala:run -DmainClass=XMLHelloWorld
[...]
[INFO] --- scala-maven-plugin:3.1.6:run (default-cli) @ scala-module-dependency-sample ---
Exception in thread "main" java.lang.NoClassDefFoundError: scala/xml/NamespaceBinding
at XMLHelloWorld.main(XMLHelloWorld.scala)
Caused by: java.lang.ClassNotFoundException: scala.xml.NamespaceBinding
at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
... 1 more
{code}

Also, if you don't try to run from Maven but just compile and then publish you won't notice any problem and you end up with broken published library due to undeclared dependencies.