I have had back problems which have been successfully been treated. I don't entirely disagree with your assessment that recumbents are not therapeutic for those who have back problems. Recumbents allow you to get away with riding without using your upper body at all. When riding my high racer I'm reclined and have to do a crunch to see around corners at intersections, other than that no upper body work at all. What the recumbent platform does for back problem sufferers is allows them to ride when they otherwise wouldn't.

FYI, I started riding recumbents because they looked like fun, not because I needed to. I still ride an upright also. The different platforms use different muscle groups so there is a kind of cross training benefit.

I have never had vibration problems on any of my recumbents, even during the time when my back was most sensitive. The three different types of seats I've tried where always padded enough to prevent this. These seats varied from reclined to very upright. The narrowest tires I've used where 28mm but due to my 250lb weight are still pumped up to 100psi. There does seem to be a window between 50 degrees and 35 degrees recline where my tailbone does get sore on long rides but that's more of an issue of my physical characteristics than a design flaw. Just like an upright you still need to adjust the bike to you.