The first thing that sprung to my mind (sprung, geddit?) is that MTB suspension is low down between the frame and the wheels for a good reason. I got my first bike with suspension (a hardtail) only a few months ago, and I really do like it - and if I did more serious MTB riding I could see myself wanting a full-suspension bike.
But I can't help thinking that the rigidity of the saddle/pedals/bars triangle is important for comfort and efficiency - when I go over bumps on my MTB, a big strength of its fork suspension is that it doesn't alter that spacial relationship and doesn't affect my balance in the way that I thought it might.
Something that moved my hands about relative to my bum and feet, or my feet relative to my bum and hands... or, in this case, moved my bum around relative to my hands and feet, well, I think I'd find that an unbalancing and disturbing experience. The decoupling needs to be between the wheels and the frame.
Having said that, I think there are some people who would benefit from this saddle spring thing - for example, people who aren't keen cyclists, but who use their bikes to ride a mile or two to school, work, shops, etc. I remember a colleague who cycled to work each day (only about 2 miles each way), and she had a very upright bike with a big springy saddle positioned relatively low, and rode with her knees bent. I spoke to her about her riding style, and she said it was comfortable for her for her short daily journeys - and I can't argue with that. I can't help thinking she'd like a Rinsten spring, and that a lot of others would too.
It's not for me (and not for most folk who use this forum, it seems), but I think it could have a market and I wish them well.
Alan