Her analogy to a hospital contagious ward captures a lot. There are degrees of good and bad in this world and communities dedicated to diversity must always sort things out. Which brings me to the best argument FOR diversity. It keeps us sharp and on our toes. We will always benefit from the widest possible range of ideas and techniques.
Diversity brings the universe to our door.
But then, just as important, is the process of deciding what to take in, what to modify, what keep out. The decision point and cut off should be as close to the everyday moral life of the community as possible. Diversity offers humankind a smorgasbord of options. We can't, and shouldn't, take them all. I also share Rosemary's preference to keep submission informal and brief.