dirklancer
Explorer
The worst example of what can go wrong with a DM run PC happened to me in a game in college. The guy running was playing his personal character from his friend's campaign at home, and he ensured his character got the best loot and most XP so he could claim to his home group it was all acquired "legitimately in play." While that level of abuse is rare, some DMs might have trouble leaving their egos at the door and be tempted to have their character steal the spotlight. While I prefer having an occasional NPC there for support, I still have the players make rolls and handle the NPC in combat so they are still in control. And if its a small group I usually have some players run a second character to round out the party's needs.