It's not necessary but it will help with ventilation and getting rid of odor. I used to live in a house where there was no bathroom fan. It had a window that allowed good ventilation. You don't need a fan but you will have to watch your walls and ceiling for mold and mildew.
A bathroom fan is pretty essential in the normal family environment if you do not have a second bathroom. By having a bathroom fan will help to reduce the smell of someones feces that they may have left behind to make it a bit more bearable if someone may need to use the bathroom right afterwards.
