I've bungied a backpack to my rear rack as an ersatz rackpack in the past. But I can't stand to wear the thing while riding. I despise sweating (when unnecessary - frequently it's unavoidable of course), and a backpack can be a culprit. I often ride with a friend who doesn't really mind sweating while riding, so I suppose it's a matter of personal preference to an extent. I use a single pannier these days. But it's a moderately impractical solution for carrying my work/spares/tools (more than will fit in any but an impractically large seat bag, but less than necessitate a pannier).