SNL female shuffling

After news last week that Saturday Night Live was welcoming two new female cast members, another announcement shows that Michaela Watkins and Casey Wilson have been fired. While I understand that this is how the show works - new cast members are welcomed aboard and others are let go - I don't understand why they chose to fire actresses instead of a few of the non-essential actors. In a show where Kristen Wiig already has to do most of the work, in my opinion, the more women they can add to the cast, the better. Wiig is wonderfully funny, but at some point her airtime will be too much.