I have never seen much downside to promoting managers as directors. First of all it makes them feel better. Secondly they take more responsibility – which they may or may not like – but you can only find out by appointing them a director. But most of all they are roped into the duties of directors under the Companies Act 2006.
Under this act all directors MUST act in the best interests of the company. And trying to plan their exit to another firm, while, for instance, taking clients away while still a director is a criminal offence. That’ll make should make them think twice before accepting work elsewhere and not telling you about it.