Many do. Only a percentage will ever become addicted to any given substance due to the necessary intersection between genetic and environmental components. "Responsible User" however tends to describe two things: 1) Someone who uses within the context of Harm Reduction, and, 2) Someone who uses and is able to maintain a licit lifestyle. In other words, supporting their usage legaly, etc. However, your definition works as well, but you may not get the desired responses. Something along the lines of, "Does Anyone Casually Use Hard Drugs?" Might get you better information.
To answer your stated question, I fit the bill as long as it is a non-opioid/opiate. If it is, I am gone, gone, gone. Addicted at age 17 and still in the thick of it. Any other substance and I can take it or leave it though I haven't done anything else since 2008 in Cambodia (it was benzos then to boost the sedative effects of DHC along with promethazine for potentiation of metabolic conversion).