If you understand all your characters perfectly you will naturally become a better writer. Since the world naturally is full of people who are different to the author then not including that diversity can only limit your writing, unless you have a good reason - ie. Jane Austen. Equally though, you have to understand why a certain person is different to you, and why the story says that has to be so, otherwise you're just tokenising. I think we agree, I'm just clarifying my point. ;)

I also have the same reaction to fantasy novels which only focus on the Good And Righteous King regaining his throne from the Evil Userpers, completely ignoring 99% of the population who live in muttle and daub, but expecting them all to come out and fight and die at the appropriate moment.