Well, you have pointed out perhaps the only real downside to ambient music - it can, does and will put you to sleep!
But for me, that is one of it's best qualities. I have done many experiments where I listened to ambient music at very low level all night long, and if done right, it can really be a lovely experience. Generally, using one long, very ambient piece on endless repeat seems to work best - I've used Eno - Thursday Afternoon, Eno - Neroli, and Fripp - A Blessing Of Tears and in varying degrees, they all work beautifully.
You fall asleep listening to the repeating yet endlessly changing patterns (most of the ambient music that I favour, and create, is "loop" based, using repeating patterns) and if you wake, or half-awake in the night, it's there to help you return to sleep - so as well as being a great adjunct to reading, normal books or e-readers, I would say that ambient music is also eminently suited to sleep to.
Why not? So you have pointed out a real quality of ambient music, it can actually relax you to the point of sleep - and in this hectic world, that's a very, very good thing I would think.
Thanks very much for writing, I really appreciate your post and your thoughts.