Why not? It's their career, let them decide. Does it effect the way they perform their job? Granted, there should be some minimum standard obviously but I don't think there is anything wrong with letting someone decide whether or not they are ready to promote, within reason.
Some of the best Mx dudes I worked the flightline with back in the day were 20 year staff types. They liked working the jet and, typically, after making Tech you start getting pulled from the flightline, no longer turning wrenches but instead filing papers. They enjoyed what they were doing, didn't care about promotion and pay, and did a hell of a job doing it.
I know this doesnt apply to everyone, but the guys I have personally seen in my time that were following this career path were usually not doing it just because they were slackers and didnt care about their job.