Just take for example 15 contestants who you face only 4 of randomly every time you challenge. And at the end, hey! It' the Champion!
But yeah re-battling the E4 is very monotonous the more you do it, at least if you have a lot more trainers with their own speciality it makes you think about strategy a bit more rather than soloing with a Lucario which is what I've done many many times in White.
The excitement is still there because you never know who you're going to face even though you may have already defeated all of them at least once.
For using E4 as a leveling place in the post-game, I would do the following:
instead of challenging the same E4, once you defeated them, you could instead enter a separate set of rooms, where you would be challenged by randomly picked gym leaders/E4/champ, all with endgame teams, while the real E4 would use the same teams as the first time.
It'd be a hybrid of Platinums battleground and the postgame E4.