If you have an external frame pack, that's what you're supposed to do--that's how those packs are designed! With an internal frame pack, though, strapping stuff on the outside tends to pull the pack off balance--they are designed to have everything stowed inside. Of course if you are using a closed cell foam pad, you almost have to strap it on the outside because of its bulk--but at least it's not much weight.

Stuff strapped on the outside of the pack does tend to get caught up in brush, especially if you are traveling off-trail. Small items tend to fall off and disappear. Even outside pockets can be a problem there.

I really did enjoy the OP--I'm still chuckling!