He may be mediocre at some things, but he is the best at other things.
The elbows and physicality is important to have, in some matches.
If we were playing Barca for example, I would not attempt to beat them at a passing game (that's suicide). I'd try and force them into a physical battle, as that's where I'd stand the best chance of beating them.
I'm not saying that Fellaini is a starter, but he is excellent to have on the bench. The Arsenal match, where we were deadlocked at 1-1 for most of the match is a great example. Fellaini comes on the pitch and immediately Arsenal's defence fell apart. Most defenders simply don't know how to handle Fellaini, especially when he is coming off the bench and playing in advanced positions.
The Fellaini and Martial substitution which I saw at the start of the season was brutal and allowed us to score 3 goals within 10 minutes.