I'll preface this by saying that I don't play Halo, but this kind of ADS will undoubtedly change the game in a bad way.
Increasing an automatic's accuracy at range will increase the average engagement distance of the game. This will end up disrupting the balance of other weapons designed to be effective at specific range, and the weapons would have to be changed to accommodate for a longer range game. The maps would have to change as well to make ADS viable, or at least balanced for the kind of game Halo is. If the engagement distance is longer, then players would be killed more often without seeing their opponent, so a player may find it more advantageous to stay in one place than to move around. As far as I know, this isn't the kind of game that Halo is.
Of course, I'm just talking about the effect of ADS in first person shooters in general. I might be exaggerating the actual range benefit ADS gives in the beta, but if the difference is so small that it doesn't have an effect, then why even have the difference in the first place?