Jay Adams in the Journal of Pastoral Practice 1.2, 1977

 “You know what I mean—do I have to say, ‘My first point today is …’?” Of course you don’t. Where in the Bible do you ever see this happen? Did Paul on Mars Hill say, “Now my sermon falls into three points today. My first point is …””

Good question. His rule of thumb is, does it help? There are few things to consider here. First, it is generally so expected that some people may just be lost if you don’t announce points. 

But on the other hand, it can decrease effectiveness and be less persausive

“But let’s say that your general purpose is to persuade to believe (or disbelieve) a truth (or error). Then there would be no special reason for announcing, “There are three reasons for believing.…” You are not concerned about whether the listener knows how many reasons there are for believing (unless that is the point); you simply want him to believe. The same is true if your purpose is to motivate: you aren’t concerned about memory; what you are after action.

Think of situations where a speaker is seeking action: he rarely says, Here are three points.

The announced outline may cause a hearer to be disengaged or to treat it like a lecture. John Owen seems to announced say 5 points and then give only 3, apparently as a means to keep people paying close attention (otherwise one might stop listening at point three because the sermon is “over”.

The announced outline may hurt as much as help.