David,
The slow rotation test is excellent for detecting pressing voids and sporadic vinyl damage at dynamic peaks due to mistracking cartridges. Those produce sharper-edged shapes in the vinyl than any musical waveform, they're faster than the fastest transient cut by a cutting head. Even at slow rpm's, where musical information all sounds fairly low pitched, flaws and damage like that sound sharp and crisp.
I haven't tried it on a record with general background noise or grunge, so I'm not sure how that would sound. Probably results would vary depending on the shape of the grunge?
Doug