If you're buying a game for multiplayer online, I agree. But for a single-player campaign experience, if a short game is to be scored down, shouldn't that also apply to all titles? This was raised elsewhere, with the suggestion games should get independent single-player and multiplayer reviews. Well, it's off topic here anywhere, so I'll shut up now!
Reviewing MP and SP independantly poses so many issues (where does coop fall? where does MP with bots fall? are video tools SP because one person uses them to make something, or MP because you publish them online for other gamers to see? is a map maker SP or is it MP because many people play it... and what if you map maker lets you create with more than 1 player? are the MP elements online, system link, offline, or a mixture of each?) it also breaks up a single product when it is sold as 1 game.
And it only caters to forum kevetching.
Reviews are based on value--not every user will maximize that value. That is why you read a review. If you don't have broadband (like I didn't a few years back--one reason I got a GCN) you should actually... read... the review to see if certain elements of the *game* are applicable.
It is all about relevance. I understand some gamers don't like MP--I don't typically like SP games because of the current state of AI to be quite frank. But then again as a MP gamer I get 100s of hours of playtime even if I skip SP. But I am not demanding SP games get arbitrarily penalized for a lack of MP. The reality comes down to value. If your game is short and repetitive it is going to get knocked--$60 for 8/hrs of value is pretty steep.
Then you have games like Bioshock, SP only also, that is 15-20 hours and has a bit of replay through a branch in the story and the RPG style of the game. And then there are games like Halo with Campaign & Multiplayer and the difference between them can be quite blurred with coop and the other tools.
My experience has been the review itself (not the number, the text) often clarifies where the value is and what type of game it is. Asking for another arbitrary number to clarify another number based on a general impression of the overall product, when the review should answer those question, only caters to a small market of users/purposes.
The fact most reviewers didn't sink Bioshock for no MP at all indicates they aren't penalizing games without MP.