I was out shooting today and kinda slid/ran down a hill to avoid falling. Had my camra on with a shoulder strap, but you still need a hand on the camera so it doesn't go flying. Would have been nice to have both hands free if I fell. I find I usually carry my backpack by hand or over one shoulder, which still leaves one hand tied up. Not crazy about sling bags or wearing the backpack over both shoulders.
Ever notice the best angle is always in the worst/most dangerous muddy spot? So I find myself holding everything so it doesn't get dirty or wet. Which brings me to the Holster bag. I have a few Tamrac lens carrying cases which I can wear on my belt and have found them to be a great solution. I have a belt case for my 70-300 so I figure I'll get a holster bag for the camera with a standard zoom/lens on.
My wife asks why I need ANOTHER BAG, same reason she has 3 closets full of shoes! The ones I have don't go with everything I own