Lets see...
I do not have to take the plywood off to insulate.
I believe I have less of a moisture problem when I use EPS foam for insulation
I think, but am not sure if it makes applying spray in foam any easier or if it gives better results with foam. I believe it does.
I also did it because it just appeared to offer me less work for the inside and it did not cost anymore to get it that way.
A downside that I did not think of was that it adds more weight. Now I have the regular plywood that comes with a trailer, but I also have the added weight for the interior paneling. But I will use the thinnest interior paneling that I can get away with. So the extra weight should not be that much. Maybe in the long run, I will have a trailer less likely to flex for whatever benefit that is.
And there may be other benefits which I am not aware of. I do know that I would do it again.