I have heard this question over and over again. So let me try and put this one to bed right now with some recent examples.
Crackdown, Assassins Creed and GTAIV are all very similar games.
Crackdown was awesome, but the mission structure sucked because all you had to do was keep killing mission selected people over and over.
Assassins Creed was an awesome set of gaming building blocks with the roof jumping and wrist blade assassination time action, but sucked because all you did was the same killing of mission selected people.
GTAIV is awesome and doesn't suck even though all you do is the same killing of mission selected people, because there is a strong narrative. I have completed over seventy missions in the game and it doesn't yet feel repetitive at all.
To me anything that doesn't have a narrative should logically be considered an abstract game. Although even abstract puzzle games like Puzzle Quest are made better with a stronger narrative. Making a game with a weak narrative is like making chicken nuggets and then eating them with the cheapest/nastiest barbecue sauce you can find. It shouldn't be done and wrecks the whole experience.