I have to agree with Over, Squid, I don't think the weapon was needed from a design point of view, but it was pretty powerfull, and amazingly annoying to go up against.
Being annoying is its main feature. They're strapped to the bottom of every drop ship in Firefight, one of the inexplicable design decisions I've ever seen.
While filling in the same category as the Brute Shot in MP, I found that gun way more fun and funtcional in Halo 3 than the Concussion Rifle.