Bobs are great if you're going to overload your bicycle, which is not always a bad thing -- as when you're touring with small children who would have a bad time if they had to carry all their own gear, or when you're not touring at all and just want to go pick up a few cases of beer.

They're also great if you're riding some carbon-and-cotton-candy road bike whose frame or wheels would fracture if any load-bearing thing were screwed into or clamped onto it.

Otherwise, racks and panniers. Do you go backpacking with a trailer?