Apologies to whoever asked this question (and the other two I’m about to answer) which I received about a month ago, just before I stopped posting… Anyway.
Well… to be honest, I’m not sure what it would add. Using items in battle isn’t a new thing (potions, X attack, etc.), so it’s not really adding a new dimension to the game, and repels are useful anyway. Also, apart from repels and escape ropes, I can’t really think of anything you could expend on an in-battle effect that wouldn’t be much more valuable out of battle: Heart Scales and shards are too important for learning moves; stones and fossils are too rare to waste; held items like Leftovers or Choice Specs will be far more useful in the long run if you keep them for your Pokémon to use – you get the idea. All in all, it just seems like a fairly inconsequential change.