Braves hardly had loaded offenses... Maddux and Glavine and even RJ threw a lot of complete games and complete game shutouts in the 90's. They're 3 of the best that ever pitched in the game, so it's hard to compare them to pitchers today.
Pedro didn't get to 300 because by the time he started aging, the over emphasis on bullpens came in play. Maddux and Glavine in their older years were above passable pitchers, Maddux was still balling in his last few years.
Just because they give MVP awards based on RBI's doesn't necessarily mean they're the most important. Baseball has evolved, and Heyward now hitting leadoff isn't going to get many RBI opportunities. That's just the reality so to say his RBI totals suck is just not fair.