personally, when i first started DI i was super bummed that the game didnt impliment the other 3 chars as AI. it seemed like a no brainer, the three people i didnt choose would be controlled by AI. it would make the game much better for those who dont play co-op, i know i planned on doing my first playthru solo (to get the story). also, if you are playing solo the cut-scenes are so stupid confusing.
"oh all my friends are magically back, cool."
if they honestly do have NPCs you can have help you out (actually help, like follow and kill things) i would love that. however, if they are anything like the AI mastermind of "nikoli" and "cho" from those escort missions... you can keep that garbage, those people were morrons and their pathfinding skills left much to be desired. the guy in that prison was the worst!