A few weeks ago, I watched the long course swimming championships. For many years, the US has had A, B, and C finals. The C finals are for younger athletes. The B finals are for the athletes who placed 9 through 16 (based on time) during the preliminary rounds. The A final is for the top 8 qualifiers. The top 2 finishers from the A final qualified for the Worlds team in each individual event.
During the TV broadcast, Rowdy Gaines made the point that the B and C finals are a good way of giving championship level experience to younger athletes and athletes not quite ready to make the world team. The US team is incredibly difficult to make in swimming – only the top two go to worlds or the Olympics in individual events, and our team is so good that getting one of those spots is a tall order (esp with people like Katie Ledecky in her prime). Having what are essentially developmental finals gives developing athletes a path towards elite competition.
I recognize that the marathon is a different beast vs. the half. I also appreciate that swimmers in the finals got there by doing the event during the preliminary rounds -- you don't make the final in the 800 free by swimming a fast 400. But I still wonder whether allowing half qualifiers into the marathon trials might be a good developmental step for athletes who are not quite ready to run a full during the window. I get that athletes with half qualifying times might flame out during the actual trials. Personally, I am OK with that, especially if it gives a talented athlete an opportunity to learn and grow, and hopefully come back.