Back about a month ago, I think it was two games again Liverpool and Southampton. In the first one we had Fellaini playing, had the lead, took him off after about 80 minutes and Southampton scored a late equaliser from a header from a set piece (possibly a corner): the sort of thing Fellaini has been comfortably defending all season.
The following game (again I think it was a Liverpool game but not sure) Fellaini didn't play and we conceded another two goals from set pieces.
After that, I suspect LVG sees Fellaini as almost one of the first names on the team sheet, especially if we play Blind at CB who is not particularly physical or good in the air.
Fellaini is an average passer, gets up and down the pitch well to contribute at both ends. He adds some steel and physicality to the midfield which in my opinion is something needed. Frankly I think Fellaini is something of a leader on the pitch - he isn't a coward, he always gives it 100% and doesn't give up when the chips are down - these are all qualities which his detractors laugh off and label as unimportant, but I have seen games without Fellaini where our team looks utterly void of any aggression, passion or leadership, and basically look like a bunch of cowards.
What I like about Fellaini in our current system is that he is able to play a more box to box role in a 2-man midfield, when compared to the likes of Carrick, Schneiderlin and Herrera. The trouble with a 4-2-3-1 when we play with two pure holding midfielders is that it leaves the attack a bit undermanned. It has been a cause of endless frustration at times this season that even against teams that we should have been comfortably beating, we play such a defensive lineup and it stifles our attack. Fellaini supports the attack, presses defenders when appropriate, but still gets back and does a good amount of defensive work in our own half.
Of course, when his detractors see this, they just say that he has no positional discipline and pass it off as a bad thing, but I would prefer seeing Fellaini getting up and down the pitch than another infuriating Carrick/Schneiderlin game where neither is prepared to get forward or leave the halfway line.*
I have always said I would prefer us to playa proper 3-man midfield like we did last season, and that he entire team would benefit from it. But as long as we are playing 4-2-3-1 I prefer to see a box to box player who can support the attack, than two dedicated holding mids.
*regarding Herrera, it is difficult to classify him, but I think LVG sees using him in a 2 man midfield as a bit unbalanced to use against decent opposition, though this is just my view of course.