My experience with the shotgun is based on 2.5, but I found it useful for more than just reaction fire. Yes, at the extreme of its range it's not terribly accurate, but it is quite damaging. Much more so than the assault rifle.
Remember also, the shotgun slugs aren't super accurate at range, but they are still highly damaging.
The grenade launcher is really good, no question about that.
Where the shotgun especially shines is in houses, alien ships, and other confined areas. Out in the open, like you said, the grenade launcher is better. But it's almost impossible to shoot the grenade launcher inside a house or alien ship due to the ceilings blocking the shot's arc.
And, the shotgun is absolutely fantastic for RF in situations where you're covering a door, or any situation where the alien has to get right in your face. Nothing can sneak in under its 8 tu shot, even a plasma pistol needs 2 tu to move into sight, then 6 tu to fire, so you'll typically kill them before they get a chance to fire.