Are you sure you finished the quest? You summoned the Fellhunter and killed it, then went back to the questgiver?
Been a while since I've done it, but I think that's how you do it.
In theory you can abandon the last level of the quest (which is killing the fellhunter, if you have done that - and it didn't work) - and redo that part (and not the whole thing). Just be very sure which quest you are abandoning (you don't want to have to redo the whole quest, so be careful).
Fellhunter is not needed in pve.
In pvp he is very useful against casters. You save the silence attack the fellh has for key moments - to stop a rival warlock from fearing you - to stop a nearly death healer type from healing himself - to stop a mage from pyroing or polying you - and so on. You have to practice to learn which spells are best to silence (go into settings and click on the setting that lets you see which spell your target is casting - helps TONS - or get a mod that does this).
GL