Promotion from within is an excellent idea: if you don't promote form within, you will slowly, but surely, start to lose your best employees. The best resources want to advance in the company and don't want to remain in the same place, and if they see that every time there's an opportunity for them to move upwards, you give it to someone else from outside the company, they will become demotivated and then they will most likely leave.
When you promote from within, your best resources will be more motivated: they know that they will have room to grow in your company, and they know that with hard work they'll be rewarded with a better position and a better pay.
Now, what if you did the right thing and promoted an ambitious and team member to an assistant PM position? Let's examine what will happen:
- If you made it clear that he's your absolute favorite and he has unlimited authority in front of the team, then the rest of the team will hate him and will hate you, and will be non-cooperative with him. Productivity will be lower, and many team members will start defecting.
- If you made it clear that he's just there to help you do the job and if you kept him in check, then his fellow team members will slowly adjust to him in his new position, and will be gradually more cooperative.
Word of advice: Before promoting anyone make sure that you have someone as good to do his job. It's not wise at all to promote someone if there's nobody else to do his job.