In my day job, I preface propositions
a lot. It's important to define what is a fact beyond change, what is substantially supported by evidence, what I think, what I believe, and what I hope.
Overstating or understating my certainty prevents clients from making informed decisions. If I inflate our chances at trial, a client could end up receiving more punishment than necessary because he forgoes the benefit of a plea deal. If I err in the other direction, a client may give up on fighting against an allegation even if we have a decent chance at an acquittal. Either way would be a failure to serve my client's best interests.
Beyond my own clients, I am also supervisory counsel over five younger attorneys. If I inject false certainty into our discussions, there are major ethical implications. If it's bad enough, it could even constitute a crime.