While it is very cool looking it is also very impractical. Shooting sticks are a much better idea. If the shooter need to lie down or quickly maneuver he just drops the sticks.
The correct use of a Steadicam. Photo from Wikipedia.
UPDATE: So it turns out this concept was used in the movie Aliens. I have not seen any of the Alien/Predator movies. Only in the past few years have I began to start appreciating sci/fi after reading a couple of the classics. Some Alien movie pictures:
“Colonial Marines Technical Manual” – Thanks to Jimmy for the scan
“M56 Smart Gun”. From IMFDB. Thanks to Linoge and Redfezwriter for the link.