Hi Walter,
I agree if it comes to assignment but not really to taking the task. It could be/is kind of a gap in the api, anyone could "steel" any task by just using the jBPM API.
I forgot to mention I was planning to do configurable, meaning that you can turn on/off such validation the same if it comes to validation if user exists or not.
Let me know what do you think.
Cheers,
Maciej