I think a broader point is that a On-Body kit is not just something that is needed for emergency's only but also for simple daily items. Face it most internal frame packs suck at pockets that are easily accessible by hand while being worn. And after market attachments are expensive.

So instead of taking off my pack each time I want a bar, seeds, skin protection, compass, whistle, mirror, map, pen, knife, etc, it is on me in such a way as I can reach it with my hands without having to be super flexible or asking someone behind me to help.

so its not just a matter of worst case scenario but also 'keeping things in a convenient location on my body' scenario. Heck I've got a Mountainsmith Lumbar pack with Strapettes that I occasionally wear front facing w/ pack & w/o cause it convenient to get into.