I don't think it would be accurate to assume all players play worse than their average in sanctioned events. I know many players who do play as well under pressure. Some play even better.
The point is to establish some basic handicap ahead of getting your first PDGA rating and for those who play local and regional non-sanctioned events.
I like comparing the player against the course, not so much against the field. after all, beating the course is usually the goal in PDGA events.
But usually that's not too hard to do. If someone has played in the area even a little they likely know someone who has a rating and can estimate where they would fit in. Then they decide to play in the next division up.