Cons:
-Competition organizers have to provide cubes. Nobody who has ever run a competition will agree to this. A large competition would require, at the very least, 20 or so EACH of 2x2, 3x3, 4x4, 5x5, and at least 10 of each side puzzle. We're talking hundreds of dollars worth of cubes.
-Even standardized cubes like storeboughts can vary greatly in turning speed. What if someone wants to try all the provided cubes and find the best one? And then everyone will want to use that one after that.
-Cubers can't practice on the cubes they will compete with. A lot of cubers would do poorly on an unfamiliar cube.
-This disadvantages a large portion of cubers, as opposed to the handful that are able to attend competitions but unable to order a $10 item on the internet, and are too lazy to get another cuber to order it and then pay in cash at the competition. That's right, your way is LESS FAIR than letting cubers use their own cubes.
Pros:
-none