Proper body alignment is very important so I do not believe that sleeping on the floor is beneficial. The same can be said for sleeping on a mattress that is too soft. People who do not have good alignment often experience a great deal of back pain. In addition, this pain can cause a loss of sleep which is also not healthy. The best thing to do would be to contact your physician or chiropractor. He or she would be able to discuss what kind of mattress would suit you the best. Everyone has their own specific needs and what may work for one person may not work for another.
I think a part of your question is missing. Is sleeping on the floor better than what? Better than sleeping in a bed? I actually don't think so. I do think, however, that generally speaking, a hard mattress is better (in the long run) than a soft mattress or even a pillow top mattress. I realize many people swear by soft, cushy mattresses - but the fact is, they do not keep the spine aligned. Instead, the put a curve in the spine in the direction of the heavy part of the body. For example, if you lay on your back, rather than keeping your spine straight (as if you were standing with good posture) there would be a natural bend from the weight of your hips and bottom. Over time this nightly stress on the spine can result in muscle pain - or worse - disc issues. The same is true for sleeping with the wrong kind or size of pillow - over time it can result in prolonged neck tension and pain.
As a result of something completely unrelated, my husband ruptured a disc in his lower back - and had surgery. Ever since then, he's sworn by a firm mattress - and he would say that sleeping on the floor is certainly better and more comfortable than sleeping on a soft mattress.
A chiropractor would tell you that the health of the entire body can be tracked back to the health of the spine - so anything you can do (starting with good posture) to keep yours healthy and strong is one of the best things you can do to protect term health. Trust me when I say that your body might likely never feel (or look) as good as it does right now. :)
Sleeping or lying down on a flat and hard surface is good for the back. It helps to keep the spine the a position that is best suited to maintain its correct shape. Persons suffering from problems of the vertebral column such spondylitis are specifically advised to sleep on hard surface without a pillow.
Of course, people with normal health will find a soft mattress more comfortable as compared to a hard surface. However regularly sleeping on mattress that is too soft will lead to spinal problems.