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.
"one could argue", "it's possible", etc. may be better alternatives for "I think".
Compare "I don't think we can fix this bug until we refactor," to "We can't fix this bug until we refactor".
Always lead with what you know factually followed by what you think.
When speaking to people, I often ask them to tell me what they know to be factual and what they think. Then I like to talk about how we can prove or disprove what we think.