I put you in context, imagine that you are going to look for a ranked game for the first time and the typical "vs Trainer" music of your respective region plays ... Nothing bad
Now, you are the same coach 5 months later with 670 points almost reaching supremacy and you listen to ... the same old music over and over again.
Now Imagine a Simple Change...
You are Fighting vs No 26 PokeMMO OU and this song sounds:
The simple drum give more importance to the battle, it feels less monotonous, despite being exactly the same composition, it gives it a fresh air.
Now imagine that you can simply choose ANY SONG FROM ANY ROM TO FIGHT.
Even a more ambitious suggestion would be to load the Assets of Pokemon Black2 and White2 just for the songs and themes of battles that you have in the championship tournament.
EVEN MORE AMBITIOUS, it would be to choose your battle theme for the opposing player to listen to, giving you one more signature to customize your avatar.
Imagine that you can simply select the theme of Cynthia being No. 23 in NU for an example and this theme plays when everyone Fight against you: