Well considering that players who move between 2 lines of defence are the hardest to defend against, that could be powerful indeed. The sine qua non condition is that when there is a cross, Fellaini has to be on the move and not wait in the box in order to benefit from this advantage.
I think it would have a similar effect if a tall striker like Crouch begins to drift deeper in the midfield and challenges for the ball right out of the box but well, as long as Fellaini does the job, why complain... I think Moyes could have done better provided he followed his guts. After all, indeed, he used Fellaini in a similar role at Everton and he was thinking of Herrera. Two very different players in the midfield. One strong player and one very technically gifted player.
Still very happy with Louis.
Well Fellaini is starting in midfield and moving forward, in defence he is a CM but when in attack he operates as a second striker almost. This means when he is challenging for aerial duels in midfield he has an advantage because he tends to be bigger than opposition offensive players but against defenders it is more difficult however because he comes from deep and so has a run on them it also makes it difficult to defend against him.
Furthermore he has a decent skill set that allows him to be useful in midfield, he is good at winning the ball back, he offers physicality and also he can play a simple passing game but also in attacking areas he has a decent shot and so can chip in with a few goals.
The position doesn't expose his weaknesses such as positional discipline and decision making, when he was played deeper he was not the best at defending the spaces (when compared with Carrick) nor at passing the ball quickly to the right player, he was losing the ball in dangerous areas.
Now for Crouch to perform a similar role he would need to be able to operate in central midfield areas and I don't think he can do that, I think most big strikers probably aren't good enough in midfield to perform the role but also most big central midfielders probably wouldn't be as effective in attacking areas. They could perform the target man role but Fellaini does more than that, he is actually a decent second striker.
Fellaini is merely one player but Louis van Gaal has found a role for him that justifies him as a squad player for a club like Man United (next season) and also a role that can make him very effective against certain teams. Carrick and Herrera in midfield is light and so Fellaini's physicality will be important especially in the bigger games but that's in defence, when in attack he can be a nuisance for defenders who will already have their hands full with our attacking talent (di maria, Mata, Herrera, Carrick, Rooney etc.).
Herrera performs a creative role but does it from deeper, Fellaini will push forward and Herrera will look to arrive into the box late where also he is effective. Louis van Gaal's preference tends to be two defensive players and one offensive creative player with the creative player operating furthest forward of the three but he has adapted and made Carrick, Herrera and Fellaini work well, in fact they all complement each other and make a good team, they split what we need between them.
Herrera can speed up our game and offers the explosiveness when we needed it, Carrick reads the game and is always well positioned, Fellaini will get stuck in and show his strength. Louis van Gaal is all about team work, that three works well together but so does the partnership of Mata, Herrera and Valencia, again all offering different qualities, even Young, Fellaini and Blind have worked well together, Young is very predictable but Blind aggressively attack the overlap so even if the defender knows Young will cut likely into on his right Blind will be overlapping.
It's a bit like ultimate team and those chemistry connections, even the relationship between the CDM and CBs, it's very important and that's why we are confident going into the big matches. On paper we do need to improve our quality of players however Louis van Gaal doesn't care for individuals therefore providing there are no egos he will build a team and he has done that, the chemistry allows us to work well, I doubt Fellaini or Young or even Mata as a RW will feature as much next season because he will have a better selection but he will still build an excellent team by making the most of his selection, the team work will be visible all over the pitch and it should have the balance he talks of, that is achieved when players help each other out, the physical players get stuck in where they are needed and the technical players create, the quick players provide the pace, those with excellent positioning minimise the space available to the opposition attack etc.