Personally I'm definitely anti-capping of any kind because as fromamerica said up there, I feel like capping is more likely to discourage new potential players than anything else. From a player standpoint, I have stopped looking at several games when I saw a cap that would have prevented me from playing the initial character I wanted, and I am usually a player who ends up picking up about four characters at a game and tries to go for diversity and balance among them in gender, personality, age, etc.
If you feel you need more of a certain kind of character, maybe try talking to your existing players about picking up more characters of that type? It's always flattering to be asked to pick up another character, and you already know that they're good writers since they made it into the game.