Actually, this kind of practice is always good for the company and the boss/manager, but whether it is good for the employee is kinda depends on the individual. If there is knowledge sharing, then the company will benefit as it means its employees will be able to contribute better, on top of that once the knowledge level is saturated, i.e. leveled out where most the employees are at the same level, then this will drive competition and again the company will benefit.

On the other hand, from an employees point of view, each person has their own skill and knowledge that gives them the competitive edge. Like you said, teaching others this is practically limiting your own rice bowl.