Someone above said that many travellers carry pacsafe backpack covers, but rarely use them. I fit into this category.
However, the few times I have needed to use my pacsafe, it has been worth it weight in gold. (Just like the extensive medical kit that I carry)
I also travel with a pacsafe money belt, pacsafe bumbag, and pacsafe slashproof, secure daypack. That is why I rarely need to use my backpack cover... all my valuables stay in my secure daypack! My backpack is generaly just full of smelly, dirty clothes...
[ Edit: Sorry, no promos please... ]