In my view, if Fellaini is not injured/suspended, we should always have him in the team (not necessarily the starting lineup):
1. Start him on the bench. If we play well and lead 3-0 after 65-70 (which is almost unheard of this season), then bring him in the final 20 minutes and give one senior player a rest.
2. Start him on the bench. If we need a goal in the final 20-25 minutes or so of the game, bring him on and play direct. This may not be pretty, but at this stage we need points. Plus, it's not that we're laying pretty any way at the moment even when Fellaini is in the game.
3. In the cups, let him play, especially against lower-ranked teams. I know it's sounds obvious, but when we're competing in four different competitions, we have to use our squad efficiently, give the senior players rest, etc. Fellaini can help there.
So, now, he's not great and who knows if we'll have him next season, but why should we think about next season when this one is not over yet?