As someone who works in the printing Industry there are a few problems.
First and foremost is Copyright Issues - Many print shops (Kinko's, Fedex Office, Staples, or even local print shops) may not print copywritten materials
Secondly as Tink said - Image quality is a major issue with DISigns - what works on 8.5 x 11 doesn't neccessarily work for enlarged images. any image taken from the web will be @ 72dpi - Graphic Designers prefer 300+dpi