There's nothing specifically addressing this issue in my manual, other than not to use a plastic bag. However, to speculate a little, I can think of 3 possible reasons, in order of likelihood: 1. Even with a "well-fitted" plastic bag, there is a great chance that the plastic may get sucked up into the cutting mechanism while operating, causing it to jam, and potentially damaging the unit; 2. The plastic, being in close approximation to the motor/grinding mechanism, may act as insulation, casuing the motor to prematurely overheat when in continuous operation; 3. The plastic might possibly create a static discharge that would effect the function of the unit (although this is less likely an issue for this unit since the product is not digital and, therefore, does not contain a microchip.)

You may wish to contact the mfr. directly to get the answer. They can be contacted at: 877-467-289, or www.insigniaproducts.com

Hope this helps.