Your frustration is understandable, I think it happens to everyone that someone rage quit in the middle of the match, and yes, that bothers.
But I also rage quit some games, (so I can also see it from another perspective) not because of the difficulty, but because it depends on the host, 1/10 matches i have 400-900 ping and the only thing that would contribute to the team would be my deaths, that's why I rage quit. and About the newbies I believe that they find the mission very hard and do not want to be a stone in the shoe