Balance is a very fluid, delicate thing though. It's impossible to effectively test all circumstances for all weapons/play styles on all maps in all game types and all skill levels. Players do that much better because there's probably a 1,000:1 player to tester ratio. As a result, weapons get balanced after lots of data comes in. One can't expect Bungie to test everything, and if a rebalance is needed, you can't expect what's changed to behave the same way as before.