It's a shame, because buried somewhere amongst all the vacuous bullshit and massive grasps for cogent points there's honestly some interesting nuggets that COULD form a decent argument.
You answered your own question with the first few words.
I'm loath to criticise Di Matteo TOO much, because I know that if Martin Laursen came back to Villa, rebuilt the relationship between fans and the club, rescued a wobbling season and then won us the Champions League against all odds and far superior teams all in one year, I'd defend him to the ****** hilt and wouldn't hear a bad word against it.
I often think United fans' views are somewhat warped by the years of success, and I think this might be one example of it. It's easy for you to say "oh it's just a CL, he's still rubbish" and "oh just because he used to play for Chelsea" but that's missing the point rather handsomely. Having a man who connects with the club so deeply lead it to the greatest prize of them all against all odds forges something of a strong bond.
Yeah, we all know Di Matteo's pretty average. Even many Chelsea fans know deep down that he simply isn't as good a manager as Benitez. But they'd prefer Di Matteo, and that's fine. It's illogical, but fine. Football isn't all about logic - and this is coming from someone who spends big chunks of his time attempting to find the logic, the stats and the hard facts about football - and we've all done plenty more illogical things in support of our club. I, for one, remain deluded that next year will be the year Agbonlahor blossoms into a beastly all-round forward capable of pulling the team one handed to a top ten finish. I'm pretty sure it ain't true - it hasn't been for about four years now - but just because I know I'm wrong doesn't mean I shouldn't stop holding that view. It isn't hurting anyone, after all.