"High quality" and "low quality" in briar have to do with the evenness of the grain, the care taken in the curing process, and other factors. Sand pits and other little flaws can show up in the highest-grade blocks. So yes, rustication is often used to hide aesthetic flaws. It can be used to hide deeper flaws, but a reputable pipemaker isn't likely to try to pass off a pipe that's cracked or patched and then rusticated.