definitely this.Finish the game.
I am not overly convinced this is necessary.
I have had stories in many things completely lose it in the last 30% (a film but I had that recently when I watched the abyss) but at the same time if I have had a bad time for the 70% then that 30% could be the bestest thing ever but I would have still had to slog through nonsense to get there which could well be a justifiable reason to tell it to do one.
I have equally had games with token story modes/campaign and excellent multiplayer, and ones I would only play local co-op (looking at Resident Evil 5 for this one). I would also happily accept a "I don't like multiplayer so didn't bother with it" (I did actually used to play COD for the story and local co-op, only time I did multiplayer was to be polite when round a friend's).
I am not going to say finishing a game is a bad thing but whether it is strictly necessary is a different matter. If you have got to the point where you can safely say you have experienced the core loop of the gameplay, or that after so many hours it has not got better, possibly also boosted yourself to see some of the end game then I am more than happy to hear your thoughts on the matter, regardless of whether you saw the end credits.