slf4j is a logging abstraction layer, you can plug in java.util.logging, logback or log4j at deployment time by providing the right jars.
So JBPM-2422 (removing abstraction layer) seems like a good idea to me otherwise we would have two layers, the one from log4j and the custom implemented from org.jbpm.internal.log, or are there any dependencies that require the org.jbpm.internal.log package?