CHIME NOISES INTENSIFY
Do you mean in the bag? Because I thought that could be done by pressing select.
ANYWAY, aside from my nitpicking, on to the OP. XD
I think their reasoning for not including these features was to make BW2 more grand and seem like it's own game. I mean, couldn't they have put in the battle frontier for Ruby and Sapphire? I think (and this is kind of cheap, but it makes sense) that they always hold back a little in terms of making the games to be as great as they're capable of, just so that the third version or sequel they make seems a little greater. I mean, all those little features add up. It's one thing to say, "why couldn't they have moving grass in BW?" but once you include all the other minor details...they suddenly become less minor. And that's what (unfortunately) makes the next game(s) in the region stand out/on their own.
Or perhaps it's just that two years later, Unova suddenly has moving grass in battle, more complex animated sprites, and individual gym music. Ah the wonders of (videogame) nature! XD