Question about channels on 'early' Trailer mixes?

[joris de man]

I've seen this with a few early trailers and segments of films; a case in point is the current 'Predators' trailer that was just announced on aintitcoolnews.
With these rough mixes, the music is mostly stereo, though with a bias to the left, and all the dialogue/sfx tend to be mono, hard panned to the right.

Obviously it isn't the final mix, so why do they do it like this? Is it an m/s process, and if so, what is the reason for it? Just curious...

Cheers,

Joe