A former manager of mine has been trying to convince me to return anytime she sees me since I left almost 2 years ago, offering me better pay and a position much higher on the foodchain. She also respects why I left (I had bigger ambitions that they couldn't meet at the time) and respects my reason that I haven't taken her up on that offer (that place only has really garbage benefits)
One of my wife's friends babysits for the director of IT at a large company 2 hours away and apparently I can get an interview with them if I just say the word.
Being professional and staying on good terms is not just for the employer, but it can also be a safety net to fall back on if things go sideways. Being able to reach out to contacts and say "Hey, I'm unexpectedly looking for a job now, do you have any openings?" is a very good place to be, plus sharing openings with former colleagues is a good way to help eachother out.