Here's my arguments:
1. It's not like they get those for nothing. I heard they need to put months of hardworks and dedications to be able to gain one. So not every staffs can get one just like that. And you're saying they don't deserve to use it? They earned it, so they have the right to use it whenever they want.
2. I know all those 3 people and it's not like they're always using it in ranked. Giovanni (Mewtwo) rarely join the ranked PvP. Taufan (Reshiram) and Clockspeed (Dialga) both have a quiet decent team without legendaries that I have fought before. I can say they don't actually need it to be competitive, they both are already good in PvP even without them, and I believe they use it only for fun once in a while. They might be able to beat you even without the legendaries, and YOU MIGHT also beat them even with legendaries, all depend on your team and strategy.
3. Legendaries also have weaknesses, I can beat reshiram and dialga with a single earthquake.
4. Without some moves and items coded, some pokes are also OP, like blissey and other stalling teams. You can use that team againts all those 3 Ubers, especially since all of them are special attacker.
5. I heard every staffs can only get 1 Uber legendary on their player accounts, its not like they can use 6 of it.
6. Last one: https://imgur.com/qLjkdhP
So if you want a pure OU PvP, wait for the tiering system which has been planned.