If I start on it now without all the files, I'll spend more time making changes later on. Since you're so afraid of chasing away customers that you won't charge for time making changes, we'll lose that much more money. Now who looks incompetent?
And when you have a customer distracted / incompetent enough to send you PART of that 'most important' job you just know they'll have lots more changes and still want the same delivery schedule and the same quoted price.
80-20 rule applies. These guys are the 20% of the customer base who we quote higher upfront.