I don't know how people could get by without one, I think it's a must-have item. Very handy for everything, I keep a Tcon, compass, matches, lens/camera field cleaning kit, notepad & pen, couple of torches, many spare batteries, spare CF cards, different eye pieces, maps, tools/knife (leatherman type multi-purpose tool), wide-angle or macro lens sometimes. lol everything that doesn't fit into my backpack. Pretty much anything I may need quickly without having to take my back-pack off to grab at things.
I'm not always in a good position (ever try to get things out of a pack on your back when in dense scrub, in water thick with reeds or up in a dang tree ? lol) to take my pack off and fiddle looking for things. Also when I'm as close to my subjects as I get I don't think such a large range of movements would be good. And lastly, I think of the ease and time saved just getting that battery or memory card right out of my pocket in a split second so I don't miss the shoot I'm after.
To be honest, I think my vest has helped me get at least half my shots by not having to mess about. I couldn't be without one, the question should be what can't you use a photog vest for lol. Heck mine's even saved my camera from the rain and mud a few times.