Saying we need less Assassins is one thing, but it's pretty difficult to regulate the frequency of roles. Take the example Yoshi just mentioned - Janitor. That isn't supposed to be a common role. It's a quirky addition to a scum faction that gives them a slight edge and causes a small degree of confusion among the town. If we saw it every game though, it'd get obnoxious pretty quickly. How do you tell someone not to include one though if there had been a recent spate of them? What makes one game more deserving than another? Then take into account that people finish making their games 6 months before they're even used.
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"Eet's game time..."