Starting an employee off right, with proper onboarding, is good business sense. It allows them to ramp up quickly and starts the relationship off well.
Offboarding matters just as much. When an employee is leaving you want to treat them with just as much respect. Not only will they remain an advocate of you and your brand, but it will make those who are staying happy to do so.
I read The Alliance: Managing Talent in the Networked Age by Reid Hoffman, Ben Casnocha, and Chris Yeh a while ago.
According to the book, instead of spending so much time hiring and onboarding staff that will at some point move on, create an alliance with them instead. Be up front about what projects and tasks are required of them, how long you think the project will last, and what you will give them in return. Once the project is nearing completion, you can check-in and see if they want to stay on for another project or role. If they choose not to stay, you add them to your network of talent and can reach out to them next time you have a need for their particular skill.
With the Alliance model, there is no hypocrisy between the onboarding message ("We hope you'll be happy and be with us for a long time") and the reality ("Your employment is at will and as such can be terminated at any time, with our without cause"). Instead someone is hired to complete x by y and has the option to move on to z if things are going well. There is still ongoing communication, feedback, and "employer" investment in the contractor's education since this is a long-term professional relationship. The only difference is that everyone knows going into the relationship how long both parties commit to and what both will give and get.
The above scenario also highlights how properly done offboarding can help the employer. If you no longer have work for a talented employee and offboard them well, there's no reason they won't consider helping you out in the future should the opportunity arise. Offboard them badly and this will not be possible.
Bottom line, always treat people right because you never know when your paths will cross again and/or if you'll need their help again.
Have you been offboarded well? What was the difference between this and a bad offboarding experience?