It's just a bullshit limit that gets in the way of my reading. More times than not I have _not_ read an article just because it's not worth the extra effort. I know the NYT doesn't care about losing my eyeball in that instance just as I don't care enough about the article to go the extra mile. It works for both of us.
Seems like an effective enhancement to freemium. The most sincere best advertising: the thing itself. And if it's not free, how will people know it's great? How will they develop a taste for it if they've never tasted it?