It is not only the unfolding but changing the aperture/shutter speed. I know it sounds trivial. I'm not sure about your experiences with cameras and how you use them but sometimes the trivial things just get in the way. I'm 35mm, I value compactness and speed the most. I love the ergonomics of my Rollei 35...but it is not a rangefinder.
What I dislike about the Retina is how the shutter speed and aperture are linked and adjusting one without the other isn't fast. I like to adjust the settings often and indepently. And this can only be done when the camera is unfolded. So when I see something interesting I have to unfold the camera, mess with the settings, focus and shoot.
With the Leica LTM I you can adjust the shutter speed and aperture independently and I would think that this can be done while the lens is collapsed. So when I see something interesting I just uncollapse the lens, focus and shoot.
The Leica LTM just sounds faster to operate.
The bottom loading is no problem at all because I do not change films that often. I have never been bothered by bottom loading my M3 and I figured that an LTM would be more or less the same...maybe more frustrating at first but I don't find it a major deterrent.