However, it also seems like a technique that you would have to practice a lot before it became ingrained in your muscle memory as a natural response in a fight, as I think a lot of people's first reaction to a punch or strike would be to shield themselves in a sort of boxing guard. I am interested in people's thoughts and opinions of this technique and how realistic it is on the 'street'.
Uke nagashi or whatever you want to call the movement (5th suburi or renzoku uchi komi in Iwama style) is a critically important sword movement. The difference between a sword or large knife and a fist, is that a human cannot cover up in response to a sword/knife strike. If that is what you are expecting to go up against, you'd be a fool to practice covering up.
It can be converted to empty-hand, but we must be careful that our movements remain appropriate. They aren't just some things that we choose to do!