These are all very interesting answers. I appreciate it guys!

If the mode was meant to be good, yes.

It depends on what the game is focused. Doom is, I agree, the perfect example. I love that game because of the campaign, so I can't care less about multiplayer. Some people might enjoy it, so it's actually a bonus.

If it's as simple as 'single player' vs 'multiplayer,' no - those two aspects deserve separate scores & reviews. If it's just one of multiple modes in a multiplayer game, it depends on how many other better modes exist. (1 bad out of 10 is acceptable; 1/5 is pushing it, 1/2...well, the one good mode had better be phenomenal.) Single player you can get away with just one mode, nobody cares about an afterthought NG+. But a good extra 1p mode can improve the score.

Of course. If a hamburger is great, but the fries expired, my meal wasnt as great as it could have been.

If MK8's Battle Mode wasnt so poopy ass bad, I would have played it more, making my enjoyment of the game better.

If I were to review a game and give that game score. Yes, I would absolutely deduct it's score for having something crappy in it. I don't think any reviewer should review games with what they enjoy in mind while reviewing. It's how reviews get bias and corrupts the validness of it. All areas of the game as a whole should count, otherwise the game is a partial game if even a single something were not counted in its score. Explain the downside in the review and never take zero points away with a downside. Even if it was nothing major, and it could only be a bummer for one person out of all buying it. Take at least .01 off from a scoring of --/10.

Relative to my desire to play said mode. Adding a bad mode I have zero interest in doesn't bother me. I have no clue how Mass Effect or Uncharted series' multi-player is as never bothered with either of those.

Conversely, I played racing games for multi-player, so the removal of that has been or would be severely detrimental to my enjoyment of one.

Depends if it's a mode I would have played if it was good. Like, if a multiplayer mode was added to a game I'm playing solely for the single player, then I don't care. Good examples are Dead Space 2 or Bioshock 2. I dabbled in the MP a bit for each, but I only really cared about the single player in those games, so the MP doesn't affect how I feel about the games.

I don't think I do. Smash for Wii U's Smash Tour is boring as hell but the core gameplay is rock solid with an amazing roster so I rate the game quite high.

Another example is tacked on multiplayer modes like in Punch Out Wii and Metroid Prime 2. I never touched these modes because to me these games are single player only experiences so I base my appreciation of the games solely on my single player play-through.

That works for me when making my list of my favorite games but I'm guessing you're asking this question from a professional reviewer's perspective and that changes things a bit since you can't really conveniently ignore part of a game because you have no interest in it.

From that angle, I guess it comes down to how you award your scores. Do you start at 100 and deduct points for every flaw? Or do you start at 0 and add points along the way according to your enjoyment of the game? I think I would personally take the second approach to rating a game so a bad mode wouldn't drag a score down, it would only not contribute to getting the score up.