Just realized I never talked about these. I think these are a gods-end for the urban cyclist. They're panniers that can be converted into a backpack. Pictured here is an Ortlieb Vario PS, but other brands make them - some better than others.
I think they're king for urban cyclists with lots of stops as you can convert them in give or take 3 seconds from one to the other (if you've not bought garbage) and have so much more comfortable carrying capacity compared to classic pannier with a single strap and you can take your valuables with you.
I agree, but the ones I mention stay on your bike and only the contents get removed so the capacity is just available when needed...well, until they get snatched which has happened to a few of my crates