I wouldn't say it's unheard of. I can recall a few instances where employees I had were not interested in advancement. Once when a manager ... my best employee was a superstar, but because of his age and situation at home he was perfectly happy with the schedule and wages he earned and had absolutely no interest in more responsibility.
Do you need the employee to do more?
I'm not saying it's wrong or right, but I know some employees who have been doing the same thing for decades and both the employee and company are content.