It seems like Lawyer 101 advice: if you are making an agreement with the opposing lawyer, get it in writing.
But I think another reason people sometimes do not send letters memorializing agreements is that they feel like it is almost a breach of the trust between counsel. We all know some defense lawyers who would shoot their own mother in the head for the slightest of advantages. It is easy to send a confirmation letter to that lawyer. (You should not even be talking to that guy on the phone.) But what about when you like and trust the other guy? You feel a little like a jerk sending out an “I’m writing to summarize our agreement today” letter, don’t you?
Do it anyway. First, people just forget. Defense counsel are often juggling a lot of different cases. They don’t remember half of the agreements they made, so the lawyer might honestly not remember. Also, even good people have selective memories sometimes. The selective memory comes out in the heat of competition and the heat of battle, particularly during or before trial when the adjuster and/or in-house lawyer supervising the case inspect the file. And…some of them you think are honest are just liars. Don’t pick the good guys and the bad guys on this one.
You should practice as if any agreement not in writing will only be enforced if it hurts your case.