[amamnn]

There is no denying that Richard Lee has accumulated years and years of useful and accurate information and experience into his books. That being said, he is not necessarily 100% right all the time.
I'm not so sure you can actually scratch a steel or carbide die with what is left on a case after firing. However, I know that junk on the case can help to hide defects, can contaminate your powder if the inside is left to just collect residue, can cause you to have to clean your dies more often, can give you inaccurate readings when you are checking neck concentricity and neck wall thickness, can screw up your annealing process, and can shed all over your nice pretty white anti-static mat you work on.
Mainly, "Modern Reloading" is a book for general reloading information and Mr. Lee is right within that context. Benchrest, serious varmint hunters, and competitors who shoot for long range accuracy will have some disagreements with what is in that book. Besides his views on cleaning brass, most of us would contest his views on flash hole reamers, primer pocket uniformers, and the efficacy of his version of the primer pocket cleaner, among other things. Still, if you're loading to plink, or hit a dinner plate sized target on an animal within 150 yards, he's pretty much on the money.