The doctrine of indulgences, when properly understood, is helpful and instructive. It certainly makes one more conscious of sin and its burden, and the necessity of pious acts ("work out your own salvation with fear and trembling, for it is God which worketh in you.")
I understood from a priest that it's not actually as easy as it looks on paper. There is a "go and do that sin no more clause". (Not to be confused with approval with all other sins.)