Kid Charlemagne
I am the Very Model of a Modern Moderator
I think my preferred solution is to have the frequently-absent players create characters, but try to steer them to classes that aren't too complicated in order to make it easier to run them as NPC's if/when necessary. If its a really rare player appearance, I think an NPC can be appropriate, and again it depends on the individual player. Some player are more open to that than others.