Nah, they're called filler mafias because noobs who didn't want to play the big games, which can get quite daunting, could play these fillers instead.
There's nfw you're ever going to fill a filler game with newbies, as there are never 12 newbies all wanting to learn maf at the same time. That's why they're open to anyone usually, as a calm, themeless transition between mafia games.
Nothing wrong with a new player getting advice from one of the better mafia players on the forum imo. It could certainly help him become a better player.
Of course not. But they should have first shot. Calling first in best dressed and having the regulars take all the spots is silly. If only one or 2 noobs want in then fair call. Whatever though, not my game.