I do see people with a single ring in place of the "middle" ring, and it just looks plain wrong to me.
You need to think about chainline, I got some singlespeed cassette spacers from velosolo which let you place the rear sprocket where you want it, so running a single ring in place of the "outer" ring is not a problem, you can always get good chainline.
A lot of SS kits simply place the rear sprocket in the centre of the freehub, which may or may not cause you an issue if you want to put a SS ring in place of the "outer" ring.