One of the most helpful things that I found, in terms of becoming a better player, was being on a team with someone who knew what they were doing. ie, how will new users know to set mode +s when they make a mafia channel? How will they know what is a believable false claim, or how to act in the thread etc.. Without players who've played a few games in the past, new players would have to start from square one every single game. There are strategies that work that most of us know of (uniting the village, making google spreadsheets) that new players won't.
I know I've been using "new players" and "new users" interchangably, but lets face it; many, many, many new players will be new users who may not go on IRC and may not be active. New players could even create a whole beginner mafia "metagame", which would be bad for new players wishing to play well in standard games.
While it may suck to have a role in your game played poorly by a new user, throwing all the new players into an isolated group could make it impossible for them to get better.