I don't think it's necessary as such, but I do think it would be awesome. Each one should still sound like a pokemon battle theme if you know what I mean.
There's a custom music pack that basically does this over in the mods section, but I've never properly got to try it out in game because it was made after my original playthrough, and it incompatible with the modified versions I've used/am planning to use for subsequent playthroughs.
I think it would be sweet if in the final version of reborn, each major character (leaders, rivals, meteor admins etc) had their own theme, though it might take a while to pick them all out. I guess it would also inflate the filesize a bit, but I doubt anyone cares all that much about that, the game is still small compared to most others.