You can see the subject even at the moment of exposure. To use an SLR one must be able to anticipate what is going to happen in the next half second. Not everyone is able to do this.
I use both rangefinders and slrs and have never understood why people say this. You always have to do this, with either type of camera. The moment of exposure means that you have already pushed the button. Seeing it or not seeing it makes no difference at that point. No mirror movement means slightly less mechenical lag between pushing the button and the shutter opening. It's a very small amount of time. I think you would be hard pressed to show an example of when it was really significant.