Then they might start putting traceable additives to foods to better track you.... eg: you eat at restaurant X.... or shared food with person Y (by analyzing combinations of additives)... or used too much of drug Z...
Especially coupled with another reply, about my insurance company buying me one. They buy me one tomorrow, and raise my premiums the next year because I drink more than their actuarial table allows.
I would 100% buy a digital medi-toilet that analyzes my wastes and warns me if something is off.