It allows to keep code up to date ( e.g. when bug is found in activiti and is resolved simulator can benefit from it). On the other side, simulator is tighten to the activiti process engine and its releases.
I started to copy files from activiti to simulator.
What I want to keep is to be at least updated in the case when something was changed in the classes which simulator uses.
Well, I don't think there's a golden egg here. It's just a matter of keeping up-to-date with the Activiti commits and see which ones you want to apply to your project.