The two (related) cases where you'd perhaps want genetic information is: a) If you suspect there is a significant pharmacogenetics component to what you are st

The two (related) cases where you'd perhaps want genetic information is: a) If you suspect there is a significant pharmacogenetics component to what you are studying (or related to disease progression). b) You're working on something preventative. From a previous job I worked at (we made PCR tests), there was interest on screening for APOE genotypes to enrich an Alzheimer's drug trial - drug maker believed that APOE genotype would have a significant impact on drug performance. But you're absolutely right that you can often (usually?) do 'enough' enriching without genetic information.