Middle name. Is it really necessary to have it?

Helen
We have a long last name with my husband (he is Ukranian). We live in Cali and going to stay here for all our life. And we decided to give our future kids short names, like 1-2 letter names. For ex. Our baby girl, I'm carrying now, will be Mo. 
But do we really have to chose a middle name? What is it for besides traditions? As I see most people even don't use it anywhere.
Can we leave it blank?