I see this argument a lot and I don't really think it holds up that well.
1. There's (theoretically) nothing stopping anyone from practicing and getting good enough that a company would want to send you unreleased cubes to test. Why shouldn't people be allowed to test something they got as a result of getting good at an event?
2. Bc everyone has a different financial situation, people who can afford a flagship cube may have advantage over someone who can only buy budget cubes. It's a very similar situation to unreleased cubes being unavailable. It's a *functionally* identical situation since for different reasons, some people can and others can't have access to certain hardware.
3. If you go the opposite direction of the accessibility problem, you run into the issue of discontinued cubes. Should something like the Valk2 be disallowed in competition since it is no longer made and available from retail stores even though it's a good cube?
And finally, by disallowing unreleased cubes, you might actually stunt cube development. Being able to use things in competition at a high level provides a lot of good info on adjustments that could be made.
So yeah, that's why I think it's a pretty silly thing to want to ban unreleased/unavailable cubes from competition.