Those rests are meant to prevent you from bending your wrists vertically, but if you rest your hands while typing, you have to be careful not to bend your wrists horizontally. Also, yes, they do restrict blood flow, even when used as palm rests. Moreover, the solid ones tend to get cold, when you aren't touching them.
If you have your keyboard in proper height and quite close to your belly (e.g. on your lap/knees), your hands should fit on the keyboard quite naturally.
All in all, AFAIK the only use, that isn't harmful, is temporary when you aren't typing, but just navigating a document by pressing few keys right under your fingers.