I'd like to view this from a competitive sense in-game, actually (yeah that's totally logical). I view legendaries as fine (Celebi, Virizion, Terrakion - all being OU), but ubers as not really due to their amazing power that nobody really stands a chance against (Lugia, Dialga, Arceus, etc).
Generally you'll find a lot of Pokemon in-game are OU because they're good - and the actual tiers represents that. However I feel that while the Elite Four tends to stick to OU or UU Pokemon most of the time (just thinking of Unova's, a lot of the Pokemon used are in OU or UU - Mienshao, Jellicent, Volcarona, Conkeldurr, Chandelure, Krookodile etc) - and generally the hardest to face in their teams ARE OU and UU Pokemon. Funny thing, most people in the Elite Four don't use ubers (mainly because they're mostly legendaries so this actually makes sense) and if you're using legendaries that are considered to be ubers in online tiering (or normal Pokemon that most people think should be ubers) to sweep the Elite Four, it feels kind of effortless, and really, Pokemon is trying to send the message of "if you work hard and train your hardest you can be the best". Using legendaries that are only just caught, or that you grinded two levels higher seems kind of unfair. I'm sure if there were tiers in-game for the Elite Four then the people who are apart of the Elite Four would use the Pokemon that aren't too overpowered otherwise it's too hard for the trainers.
I dunno, to me it sends the message of "catch a legendary and beat the Elite Four then you're the best!!" and really, catching legendaries isn't as hard as RNGing (I'd assume? lol) or EV training, or breeding the Pokemon to know the right moves, etc. I feel that's much more rewarding, and since it's a children's game, I do think that's the message it should send. I think it should be a challenge - where you actually DESERVE the Elite Four title.