I like having options. Haven't owned a backpack in almost 20 years, but I have a huge pile of messenger bags, rack bags, panniers, and assorted crap all over my apartment. Last few days I've been using a Carradice saddlebag for the commute, a few days before that it was one pannier and a handlebar bag, week before it was a messenger bag. The bike I'm riding has both front and rear racks, and sometimes I feel silly using just a mess bag or a saddlebag, but who cares. All three of my bikes have racks, two have both front and rear, I like being able to pick up a package from the post office, a case of beer, and some takeout food at the same time. At some point soon all three bikes will be fully racked, front and rear.

Of course, of all the cars I've owned, my favorite ones to drive were cargo vans. So that may help to explain my perspective. I also don't find the differences in handling to be an issue, don't care at all about putting more weight on my bike, and think that the more clunky crap that's attached just makes the bike look more useful, therefore better. I do lock my Tubus rack to the frame along with my rear wheel but if someone really wants to steal a cheapo aluminum one off my other bikes, they can have at it, as long as someone gets some use out of it.