If the reg is stuffed on the bottle and sidemounted you can lose a significant amount of gas if it happens to get knocked while the mouthpiece is facing up. Same reason why deco bottles are stored pressurized but off.Didn't think that I would need a shutoff with the bailout valve in plain view on a sidemounted tank. I also check the BO SPG pretty frequently, and I don't think that a free flow would be unnoticed.
If you clip the reg forward and in a way that keeps the mouthpiece down this is a lot less of an issue.