Depending on situations, I have always let the worker to leave. I mean it is not worth it trying to spend more money to sue a staff because they left suddenly. I see that situation as a challenge for me to prove that I can raise another capable hand that will be in control of things at the office and spending time to chase shadows doesn't cut it for me.
There's a very good point in what you have just said and in the end, if the money they will recover from the staff isn't more than what they will spend in legal fees, it's definitely not worth it. It's just going to be a complete waste of time and resources. Unless the company just want to set an example with the worker.