Idea is to allow flexible configuration to have a choice of what you would like to use if it comes to various types of activities. Not all of them are work items and abstract task is considered as extension point for domain specific services. jBPM will provide more and more work item handlers that could make your choice easier by just selecting one instead of implementing but that takes time and contribution is always welcome :)
For documentation about them take a look at jBPM docs and BPMN2 spec.
HTH