Quick Answer: Someone who has never adjusted his difficulty settings.
But what if you get on a great team and gain 2 levels before you get back to that mission in your list? at +0/X0 its too easy now. So you bump it up. Lets say +2/X0 now its should be the same difficulty to you as you left it before you gained 2 levels in that great team.
But lets say now, you jump on a new character, you want to lead a team, and you have 3 good people who want to join you. Your 4 man team can't run at +0/X0. You bump it up to +1/X4.
You breeze through that mission, bump it up to +2/X4. Next mission seems just right. You thank your team and log off.
The next day, you're going to solo a bit. Find that +2/X4 is just way too hard, almost died in first group had to eat 2 lucks and all 3 of your health inspirations so you wouldn't die. Decide to drop it back. Lowest you can go now is -1/X1 player, that's too easy. Guess it's 0/X1 then. You wonder, but never ask, how come you can't go back to +0/0 again?
What does that really mean anyway? You thought you knew, but do you really?
I always thought 0/+0 meant to run the mission the way it was originally intended, before level adjustment was a thing. I suppose on most of the old content, that you see when you visit contacts the old school way, it does. The new mission arcs I wonder if there's any difference between +0/0 Players and +0/1 Players? Is there?
Sorry for the long rambling post. Here's the ;tldr version:
What does +0/0 really mean and why can't you go back to it once you've changed it? (Zero Team size means what?)