The distinction between mindful and mindless is about curiosity. Harvard mindfulness researcher Ellen Langer suggests that unquestioned rules can make us more mindless. Curiosity makes us more responsive to context differences and changes. Rules make us less curious and therefore less responsive.
The practical utility of rules is that we can go into automatic. We only need minimal attention to context. The whole point of a rule is to be able to disregard distractions and temptations otherwise.
The both-and is that we can be intentionally conscious while following a rule. This allows us to keep rules relevant to shifts in reality. As rules represent one way to do something, being mindful opens creativity that reveals even better ways.