In recent months I've taken the rear racks off two of my bikes and have been using backpacks, even in the hottest months. I have a large collection of them, and they all get used at one time or another. Most are capable of taking a laptop. I've never enjoyed using messenger bags.
I kept a rear rack on one bike however, and that comes in very useful whenever I need to sling on some panniers and/or rackbag for extra capacity.
I don't own a car, so it's good to have options.