I think the main reason I'm so sad about the Battle Frontier being cut is that I never got to play through it as a kid. I battled a few Trainers at some of the facilities, and then corrupted my save file by trying to clone a Rayquaza. Never had the chance to face any of the Frontier Brains.
But oh well. I guess I can start a new save file on Emerald and try to complete the Frontier for the first time.