The thing about fellaini that keeps me wanting him around is this: if you were playing against him, you'd absolutely hate your job. He's physically huge, plays the chippy game, and is a consistent aerial threat. If you ever played soccer you always had one of these types on your team, and you know how he/she always got inside the opponents head.
At the end of matches, when the other players are all tired, seeing a player come on with his physical dominance and aggressive mindset really lights a spark.
Roberto Martinez realized this and played him a lot in the WC. He's not the best technical player, but he damn sure has his usefulness.