Nope, there's no reason to turn it into the kind of childishness that you see in political ads. "Person X says Person Y does Z which is a crime against man" should be reserved for when you have nothing good to say about yourself.
Negative Ads, while distasteful, are often very effective. But, I understand that MS or others might not want to stoop to that level, I don't see any kind of campaign / promotions / etc. that even challenge their assertions, not counting the C9 / on10 videos.
Shoot, If I had a company which made product X, and my competitors spent millions on a media campaign to say it does something bad 100% of the time, when in fact it didn't, I'd be suing their Aszes, if for no other reason than to pull the ads off the air.
If Microsoft doesn't speak up, many will just assume the ads are correct.
That being said, the proof will be in the pudding, so to speak: let's see how Apple does next quarter.