I'll admit I haven't looked into QR codes very much, but I imagine they're not robust in the face of defacement. For a standard bardcode, if part of the barcode is torn, smudged, covered, or otherwise occluded, there's still a chance that the scanner can read enough the barcode above or below the damage it only needs to read a "line" across the 2D plane that is the barcode. With a QR code, I imagine that it's so data dense that a small amount of damage will render it unreadable.
Hmm, just did some reading; apparently they can handle 30% loss of surface area. Compare that to UPC being able to lose 99.99% of surface area (at the proper angle). I always see UPC barcodes torn, crumpled, inked-over, because they're usually on the bottom of a box or bag. They'll need to splat these QR codes on multiple sides of the packaging to be safe.