Personally, I'd rather have it in one centralized location, like in Emerald. My reasoning: Contests require preparation (i.e., giving Pokemon the right moves, the proper Pokeblocks, etc.) They're not something you just cruise into town and compete in with your in-game team. And in R/S, I had one heck of a time remembering where all the Contest Halls were, and having to fly back and forth between them was a royal pain.
Multiple contest locations might work for the anime, but in the games it's much more convenient to have them in one location.