As a starter in a more or less standard midfield capacity (DM-CM range), Fellaini has been mediocre for United, I don't see how anyone can deny that. Not quite as useless as his harshest critics claim, obviously, but that seems beside the point if the question is whether he's worth keeping on the books. You don't go from "he isn't a pub player" to "let's give him a testemonial".
One has to look at his performances as a specialist, then. He has contributed to results here and there, undoubtedly, both as an offensive and a defensive impact sub. But he's hardly done so to any remarkable degree. The question isn't whether he can, potentially, be of use in a given match scenario - but, again, whether it makes sense to retain his services.
The "everyone can use a hoof target" argument certainly makes no sense. We used to send Schmeichel up into the box as a desperate measure, and he even scored once, but that's no argument for hanging on to a player whose standout qualities are mostly useful in a scenario which a top team should be able to avoid most of the time. If you need a hoof target, you use whoever is most suitable (see the Pique example above), you don't pay an otherwise utterly replaceable player top coin in order to sit on the bench as a permanent "Plan B".
At least not when the player in question isn't remarkably, uncommonly effective in said capacity.