That's an approach I've always disagreed strongly with. If a game comes with separate components, I'd rather that the better one reflect its official score, since you're in fact getting that game in the package. There is no obligation to play multiplayer in Tomb Raider, in fact, its mere existence is just a bonus when you look at it that way. It's there if you want to try it, but it has no relevance to the single player. "Scoring the game as a whole" just doesn't make sense when, as a whole, there are literally 2 games on the same disc that are completely unrelated, one being a million times better than the other. If they were intertwined, then things would be different. Few games do that sort of thing, though.
I mean, what's your favorite game of all time? Imagine it had some bullshit tacked onto it, like adding Horse Racing Mode to Ocarina of Time. So, now your originally 10/10 review is notched down to a 9.5? That's just a petty, confusing, and absurd practice, in my mind.