Our gym has always done this. As you said, they still have to score out of 6 eventually (our prep ops usually do the first meet of the season at level 6). The progression (theoretically, assuming that they score out of level 6 and get their level 7 skills) is level 5, beginning prep-op, intermediate prep-op, 7. It gives them two years to develop the skills that they need for level 7 (DD is at a YMCA gym with limited hours) and the the coach has always maintained that level 6 is brutal.
That said, my daughter is part of a "guinea pig" group... the first group at our gym to do a full year of level 6 (assuming she gets her level 6 skills before the beginning of the season) and the coach doesn't plan to have her group do prep op at all (they're all younger than the previous groups to reach this level... all still in elementary school). I have mixed feelings about THAT. On one hand, I like the idea of her sticking with the numbered levels. Some girls at our gym seem to get "stuck" in prep op. On the other hand, with each progressing level, the difference in hours between the Y gym and the private gyms grow bigger and bigger. Prep Op would help to level that playing field a bit. And also, I worry about her NOT getting her level 6 skills, having to repeat 5, and then losing all of the level 6 skills that she HAS developed over the summer. If she did prep op, she could use the level 6 skills that she's learned so far, and potentially even add new ones into her routines as the season progresses.