Hi Powdergun,
It really comes down to what your comforatable with. I fired one of Popurhedoff's sniper rifles and on the first shot managed to take the primer out of a 12 gage shell at a couple hundred meters (from a bipod, sitting). That is not because I'm so good, that is because his rifle is "perfect".
Yes, but try duplicating or triplicating that shot with a bipod. Bipods are great for hunting and snipiing, they are not consistent enough for target shooting and tack driving. The logic as others have stated is that there is always a little variation. Heavy rests allow for a constant which means less variable movement which allows for increased accuracy. Preferewnce is one thing but accuracy can be best achieved by a heavy bench with baby or talcum powder to allow for consistent recoil slide.