I use a SS as my errand bike, and its max capacity is two rear racks + a full backpack on my back.

One gear is fine for me because I live in a pretty flat place, but if I were to go with "real" cargo loads (e.g. Big Dummy style), I would definitely want some gears.

My ideal cargo bike would need only three speeds though (especially in a flat city): empty, heavy load, super heavy load.