Specifically on topic, when you pass a weapon to another, it should be verified clear and cold by the passer and the passee, as has already been said. I agree wholeheartedly with that. (I also understand the mentioned 'combat exceptions', when it's more important to shoot than be 'safe', but hopefully most of us don't have to see that circumstance).
I once had a hot M4 passed to me barrel first by an idiot with the safety off and no trigger discipline. I'm a lucky guy. Shooting or thrashing the dork was not an option, but I never was around him when he had a gun again. I understand what most of you are saying, and I apologize if my previous post sounded a bit harsh, but there are circumstances that you need to learn to live with. I don't consider a holstered weapon's muzzle a 'sweeping' event. I couldn't, and can't. It's just not reasonable, particularly in a military or combat environment. And not even in a 'civilian' one, unless we 'ban' just about every mode of carry except stong side hip or thigh.
Steven1281 has a point, as well. The muzzle of any holstered gun is always pointing somewhere. There are more rules than one, and this is one reason why. JMO, based on my experiences. You may disagree, that's your perogative. But I don't think you are being practical, at times, and this obsession about the 'unsafe' sweeping with shoulder rigs is one of them.
Having said all that, I don't use shoulder holsters.