The unspoken advantage of the pistol is that it takes up a small amount of screen real estate, allowing you to see more Meanwhile, the DMR's FP model and muzzle flash is so obstructive, you can easily lose your target in close quarters. Plus, I think the pistol has a higher rate of fire, making it even better suited for landing a desperation headshot at close range.

And the reason we don't need an SMG is because we have a shotgun. Bungie describes their combat roles in terms of range and damage; "bullet hose" isn't a combat role, it's a theme.