My opinion on this is always that it doesn't matter if a kid on a team can throw level 6 skills - they can still only do the skills allowed at the level they are competing in. Nothing says the back walkover of a level 6 athlete is better than that of a level 1 athlete.
It's true that even a Level 6 crossover could possibly blend into a Level 1 routine without noticing them, especially if it's a last minute emergency replacement. You don't necessarily need to have a great back walkover to make it in the upper levels. If you asked many Level 6 kids to do a back walkover right now, they might not be able to do a good one.
However, there's no doubt you can sometimes manipulate a routine with crossovers, which is why some bigger gyms use crossovers regularly and already have them in their routines at Tryout time, not just when they need a fill-in. Take a typical Level 2 athlete and a Level 6 athlete and have them do a series of back handsprings, and you'll often see a stark difference in the quality, speed, and power of the pass. A level 6 athlete doing a Level 3 tumbling pass might be able to fly higher on her tuck and have more power in general.
It especially looks out of place when Level 6 male tumblers are in Level 4 routines; true Level 4 boys don't stand a chance. There used to be a gym in my area that had a Jr 4 flyer cross down onto Youth 2, and she looked so blatantly out of level that it looked like cheating. Yth 2 teams are usually filled with beginners who just learned the skills and are less physically strong, and you can often see a real difference between them and a seasoned Jr 3/4 kid.
I wish there were more crossover rules. Crossing UP a level might be okay, but crossing down often isn't. There's a gym in my area that uses some crossovers when they absolutely don't need to. They let kids choose if they want to cross down to another team (most kids don't because they don't want to get burned out, but some do), which it turns beefs up their rosters, but their rosters are already large to begin with so it feels more manipulative rather than necessary. The worst is when the same kid is in the center of two routines of different levels. It's one thing to need a fill-in or replacement, it's another thing to be blatantly trying to gain an unfair advantage. I'm sure there are true Level 4 kids galore on 4.2 teams.