1) Demo'ers: They try before they buy. Publishers still might make money off of them but only if the product is good and there isn't something (like over zealous DRM) that keeps them from buying. DRM can prevent a sale in this group.
2) Causal: They obtain the material and never pay for it. If they can't circumvent the DRM, they throw it in the recycle bin and that's that. DRM has no impact on this group.
3) Criminals: They obtain the material (may or may not be purchased), mass produce conterfeits, and sell it for bargin rates making a fortune off of it without the publishers seeing a dime. DRM has no impact on this group (at least once it is defeated).