So, here's my issue - if you really liked the campaign but didn't like the MP, shouldn't the strong campaign give the game a higher score since it's good enough on its own to sell the game to people?
For example, Dead Space 2 is a fantastic single player game with bad multiplayer.
Greg Miller admits in his review that the multiplayer "did nothing for [him]" but he still gave the game a 9 because of the strength of the single player. If he scored this the same way you scored Doom it would've been like a 7.5.
If one part of the game is strong enough to warrant a purchase, I feel the score should reflect that.
edit: To be absolutely clear, I respect your opinion and I totally get that the MP just isn't your cup of tea. I just disagree with the score your gave it based on your review.