I think you have to look at the needs aspect of it. I just completed 1000 miles with lightly loaded paniers, camping, and lots of variable weather 35 centigrade to near freezing. With a basic load including the paniers of 35 pounds, I would be hard put understanding why a trailer would be better. One advantage to the trailer is the fact everything is in one location, with paniers there is a lot of opening and shutting going on. Plus a bob would allow interesting packing options. But overall, on this level I don't see any need for a trailer.
When it comes to carrying kids who eventually weighed the 100 pound maximum, we used a trailer, it just made sense. So the same would apply to the carrying of either oddly shaped or just plain mega loads, like say going into the backcountry on a hunting trip. A lot of people carry as much as 80 pounds, and some say that a bob and front paniers are better than having anything in the rear paniers at those load level.
I'm just saying, rather than looking at apples and apples, I think the reason for a trailer is obvious, rather than being a prallel thing to paniers in most cases, it would really apply when the loads get out of size, and there can be real reasons for that. I don't buy that one can't control the accumulation of gear. Just start with the assumption you aren't bringing everything that you may "need", set a weight budget, and live with it. I had probelms with the gear I didn't bring, but I was not going to go nuts on the weight thing, as a majr priority. Bikers and hikers are gear freaks, that is cool, but the point is having fun, so you need to be willing to carry a light enough load if hevier weights are going to ruin your trip. If you can carry the hevier loads, then it is up to you.
Trailers are also great if your bike won't carry gear, like some recumbents, or sport bikes.
By the way, you can manufacture a good trailer for peanuts, there are web plans. Yes the pricing on the BOB is obscene, but it is an easy piece of gear to fab, so I don't see that as a problem, at least for me.