Yeah, my reason for wanting to monitor in stereo is basically what runonce said. In addition, I'll be using a Sennheiser MKH 50 for the mid, which is a little narrow and reportedly can sometimes create some weird, blank spots in the stereo image for some sounds. It's pretty easy to fix, just by getting the right distance from your source but you can only do that if you're listening to a decoded stereo mix.
The reason I'm recording it as two discrete tracks (the mid and the side) is so that I have total control *afterwards* of how wide or narrow I want the sound to be. If you record in stereo you're stuck with whatever amount of width you've chosen and I don't want to chose that in the field, because sometimes there is no *one* correct amount. For film work, sometimes you want the exact same sound in two or three different perspectives to match different shots in the film.