illustration (attribution, if any possible, is at the end of the article)
"Mindfulness" is different from "mindfulness of " anything specific, arbitrarily chosen. Being mindful of e.g. your words is useful, but it comes with the idea of controlling your words (rather than observing and seeing how conditioned consequences arise), so what most people consider "mindfulness of speech" actually falls under the heading "virtue" (sīla in Pāḷi), not under the heading of "just" mindfulness. The mistake usually arises when people talk about using mindfulness in order to improve one's life (in particular using mindfulness in a therapeutic context, in self-help or self-actualisation).
Utilitarian perspectives of mindfulness or meditation (practicing in order to get something or become somebody better) is still missing out on non-self, on non-attachment... and on mindfulness itself! One cannot understand non-self while clinging to the idea that the self would be enhanced by such understanding ;-)