As Blackguy, Dorganath and others said, common sense is the key.
As with all "rules", it's more important to follow the spirit of the rule than to follow it word-by-word.
Strict rules for everything has its charm, as there won't be any discussion: "The rules says so and so be it."
But if we players are responsible enough such rules don't have to be made. There are most probably certain (extreme) circumstances that could validate the presences of a "wrong-level" character but that shouldn't be an everyday thing.
Dorganath brings up a very interesting thing. The RP reason he mentioned would be a RP reason. But if you are the player behind the character, take a deep breath and think first: "Is this really a solid RP reason? Or is this more like a RP excuse? Am I finding this reason/excuse just to come along or am I doing it in true RP spirit?"
If we think this as a lake. Are we building a dam to change its course or did it change course naturally?
Disclimer: This post is surely not directly at someone or anyone really. Just some thoughts I thought might be nice to share.
