[quote=Mtntopview]
How vital is it to have commoners?
[/quote]

Humans are often what make the story interesting. The heroes of the story need people to interact with.
If you create a character and name it that means you have plans for them in the story.
If you introduce a character without naming it, you can see that as an npc. Anyone can control these as they are part of the surroundings rather than characters in the story.
A crowd of people running away, a merchant trying to shake the hero of his many, the old priest on the chair at the festival.