In some use cases, like the Camel CDI extension, some classes are excluded in the beans.xml and may not be in the class path. In that case, that leads to having a ClassNotFoundException thrown by the adapter that tries to load the bean classes regardless the exclusions from the bean archive descriptor:
https://github.com/arquillian/arquillian-container-weld/blob/d8f69bed899ed027ab271ba1c83a07ea51f58eb8/src/main/java/org/jboss/arquillian/container/weld/embedded/Utils.java#L151-L160