I can sleep most places, but I like to have the temperature just right and something soft for my back (just got back from backpacking, my toes were freezing, and several roots/rocks were digging into my back. Did not sleep very well.)
But, I like a bed raised above the ground so I can slide out and immediately be on my feet/in a sitting position. Also, beds provide a lot of freedom of movement.
It can be difficult/painful/hard to find the motivation, to get off the ground for me.
I also can't sit on the ground without a backrest, or else my knees and/or back will hurt.
Hammocks are alright, but semi-restricting. (you have to sleep in them right. Lying like a banana is really uncomfortable. Brazillian hammocks are the way to go, you lay diagonally or horizontally.)
I have a big foof-chair (bean-bag chair with foam inside) that is alright to sleep in, but also restricting.
I used to have a metal bunk-bed(well I guess it was technically a loft-bed). I hated the thing! I didn't like having to climb up and down, I kept hitting my head when trying to use the desk under it, and I also hit my head on the ceiling, and the thing squeaked every time i moved!
hmm, water beds are comfy (or at least it was the one time I got to lay on one when I was 6 lol) . I think a suspended bed would be kind of cool.
In answer to the OP, My bed isn't a waste of space, I cram stuff under it, sleep/lounge/sit on it, and it is a workable surface that is usable while standing.
lol, to add to this already over-long post on sleeping spaces, some bed-related links leading to pictures of interesting and possibly comfortable beds.
Now I am going to bed.
http://www.vat19.com/blog/2008/06/delightful_strange_unusual_bed.html