Now that's interesting. I think the soft brush will mess up the drybrush work easier, and it's difficult to get the right amount of paint on it. It either leaves no paint, or too much. But they can be used on some cases though I'm guessing, and it's up to getting used to it propably. I'm in no doubt it works for you.
But I prefer stiff brushes, as you said, like people usually do. The Games workshop brushes are pretty good actually, only thing is that drybrushing is quite fatal to brushes, so if I need a brand new sharp one, I have to get a new one often. And at least over here they cost IIRC about 4.50-5 euros.