I'm not sure i'd call that a discussion in that thread, hehe. They seem to be saying that interlaced video fields and motion blur are the same thing?! bless.
The motion blur in crysis won't mask a low framerate in terms of responsiveness, but it will change the perceived smoothness of movement within the scene at low framerates. Whether that's moving the mouse (the entire perspective) or just a guy running across your view.
In short, it'll give the impression of a smoother framerate, without the same responsiveness improvement you get from a high fps.
I'd imagine it makes a huge difference to perceived fluidity of the game in the 35-to-50 fps range though (will appear more like a solid 'gaming' 60 fps to the eye).
I'd say it's a good thing, and it'll help immersion even when you've already got a great framerate.