I have a rack on two of my bikes ATM. It's really hard to bunnyhop with a rack and anything in bags on it or hanging from it, so that's why I personally dislike them. There's no way you'd ever catch me fitting a rack to my Pinarello - that's just WRONG! So I'm kinda like Steely, but unlike him, there's no way I'd not have at least one bike with a rack if I had at least 3 bikes.

Backpack is easier for multi-stop shopping trips in higher crime areas. Wheel up to next store, and your stuff is off your bike as soon as you are.

As to your U-Lock concerns, mine fits nicely in a rear flap of my back pack. No unzipping required to access lock.