That is what it does. But I was suggesting a custom option where pressing it would toggle the preview on and the aperture would stay stepped down until you pressed the button again.
But for what use? To always have a dark and unusable viewfinder?
But to be honest, I can live quite well without the DOF-preview. For me it's a nice feature for beginners, but if you have photographed a little, you'll know how it'll look (approximately).
But perhaps that just my fault having learned in the middle-ages where pictures still were analog ...
But for what use?? Of course it has a use. It steps down the aperture so you can see how out of focus the background will be at f5 or f8 or any F stop. Shooting at f3.5 on a f2.8 lens and need to see if you have enough DoF to see if the subject's eyes and nose are in focus but still make sure the trees in the background are acceptably out of focus? Use the DoF button. Want to see how low of an F stop you can get away with on a landscape shot? Use the DoF button.
This is simple stuff. All DSLRs have them. And every film SLR I've used has DoF button too so it isn't anything new on a camera.