Halo 2's way was the best because all you had to do was hold down the grenade button or press down on the d-pad
With the ordinance drops and such, i can see how that'd get in the way, unfortunately, there is no other button because every button is something important, there's the up button, but that's probably voice chat as always
Thinking about it, i can see why they have this combination thing, if they use a two-button combination, during offline custom games, you can accidentally press two buttons at the same time thus lowering your weapon which may interfere with a battle