The other big problem with AI only maps is that as the map maker there are no surprises. The map will play the same way EVERY time because you know where the AI are and what they'll do. At least when you made a MP map you didn't know WHAT human players were going to do. You could only try to direct the action but there's still unpredictability.
[EDIT] Ugh. I just watched that video and it makes me absolutely sick. It looks absolutely AMAZING and I'll never be able to make the maps I wanted to with it. Once again to the ******* that decided to cut MP: **** you!!
I pre-ordered the game and am glad I did but I vow that I WILL NOT SPEND A PENNY ON MAP PACKS FOR ANY MODE UNTIL MP MAPPING IS ALLOWED. This is from a long time Far Cry fan who can appreciate the potential of this option. I am also not a mapper but feel cheated by not being able to play custom co-op maps and MP maps that others have created.