Well...Burnt powder makes carbide which is harder than the steel of the barrel and enters the barrel behind the bullet. These particles of carbide can stick to the rod during cleaning and can be very abrasive to the bore of the rifle as you scrub the particalized rod through the barrel.
But you'd think the carbide you speak of would be fire lapped into the bore on successive shots if that were the case. Most barrel erosion comes from the high pressure of the round just in front of the chamber and failing to use a bore/chamber guide, and/or incorrect sized cleaning rod (flexing).
I seriously doubt much scratching or erosion comes from particulate matter collected on the rod. Yet again, I say it's a good idea to clean the rod frequently. JMO, YMMV