I'm one of the people who voted "on a regular basis", but that doesn't mean I track every passing boat. It just means that I regularly find situations where it's useful.
At night is an obvious one. So are low visibility situations like fog.
But I have found many clear visibility situations where I find it very helpful. I've been in plenty of situations where I am on some sort of intersecting path with another boat. Of course if they have AIS that will tell you all you need, but many boats don't have AIS, and lots seem to think all they need is a receiver, but that's another discussion. Anyway, getting a MARPA lock lets you easily sort out how much of a collision course you are on, and monitor the situation through a safe pass.
I have also had situations where I'm working my way through a fleet of fishing boats with gear out running a depth contour. A MARPA lock makes it much easier to see where you are clear and where you need to make a course adjustment to stay clear. Maybe others can judge all this visually at a long distance, but I can't at the sort of distance that I like to keep from other boats. MARPA lets me do that.
Even on the ICW I have found great use for MARPA. I have encountered other boats that I am overtaking or that are overtaking me, and a MARPA lock lets me see how fast they are going. With that, I can figure out where we will pass and be sure it's a safe location, or adjust speed to cause it to be a safe location. I recall one particular case of this on the St Lawrence (not the ICW) where we were being overtaken by a freighter while passing through a particularly narrow and windy section. By knowing it's speed we were able to figure out where the pass would occur and see that we would be past the narrow section in into an open area. It took a lot of stress out of the next 45 minutes.