A couple of reasons. I have 3 backpacks. The one I've been using the most doesn't have a good place to clip a light. My Dakine is all black with a busted sternum strap. And the one I use the least is a Banjo Brothers- I find it too big and lacks the organization that I crave (tempted to put this in the 'for trade' thread to go along with the Banjo mess bag that I'm no longer using).

Also, my Cherrybomb has a weak clip. I have had to backtrack to find it on a couple of occasions when I tried to clip it the the Townies.

I've actually removed the rear rack, thinking that I would order an adapter that would let me mount the 4D to the seat post using the binder bolt. Now, I'm getting ready to put the rack back on and see if I can tolerate the weight on it. I'm going to use my Dakine on my back in the mornings (ride the bike all of a block then), but shove it inside a waterproof stuff sack and tie that down to the rack for the rides home. My biggest fear is that the screw heads will chew up the fabric of the stuff sack.

The seat post light is going to stay there. The rack light will stay on the rack. I just need to get another one (or two) to mount on the rack. I figure by doing it this way, whether I have a load on the platform or hanging off of it, there should be at least two lights visible at all times. And mechBgon pointed out some lights that I can insert into my handlebars.