I'm pretty sure the reason they are doing it this way is because the matchmaking mechanic is already in-game. They just switch the mechanic being used by the Weekly Heroic. Giving players an option means building a new mechanic, which requires more work. Simply put, Bungie took the lazy way out on this one.
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I've no idea how to code, but it surely cant require More than a few lines to give the player the option to turn it on or off. Obviously I stand to be corrected.
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You could be right. Regardless of the amount of extra work, its still extra work. I work in software development, you would not believe the number of times I see devs re-use code to accomplish a task, even if that code isn't the best fit for what was needed, rather than build something new.