Well, Yahtzee definitely had valid points for not wanting to play multiplayer.
The Greater Internet Fuckwad Theory argument was one I certainly expected and can understand. It's less prevalent on the PC, but even there it's visible in spades. I hadn't considered the Australia connectivity issues, but I can certainly understand, considering that The Escapist's TF2 servers are in the UK and I'm in Canada. I also agree that a game which has a single player mode should be judged on that mode (if they don't want to put the effort into the single player, then it shouldn't even be there).
But I will vehemently insist that a game like Modern Warfare 2 should have it's multiplayer judged. Though the single player game is there, and also needs to be reviewed, the multiplayer is really the focus of the game and certainly accounts for its longevity. In many ways, the single player is just a big tutorial for the multiplayer game.
That's not to say Yahtzee needs to review it. After all, despite doing reviews, Yahtzee is largely a comedian and entertainer. If you want a thorough, unbiased review, look elsewhere.
But I do think it would be worth your while, Yahtzee, to try out the multiplayer. Sure, you'll have to deal with assholes, but if you use the typical level of asshole as a baseline, you can at least judge how much fun the multiplayer can be despite assholes, or for people who are used to assholes. In fact, a big argument with the whole "no dedicated servers" debacle is the players' ability to deal with assholes. It would certainly be worthwhile to see how this holds up in MW2, especially compared to the original CoD4:MW.