I think panniers are more akin to the trunk of the car. Whereas a a basket is like the passenger seat. Where you can throw stuff you need without being too careful if you will. The basket is easy access, so if I am wearing a sweater and get hot, I can easily take it off and throw it in the basket. Or if I just need to grab a few small things.

I use a rear basket for groceries but I have a CSA and sometimes stop after work, so I don't use my bike for groceries as much. If I carrier a pannier all the time, I'd have to lug it everywhere and that's too much for a trip to the pharmacy or the coffeeshop or happy hour!

So they totally have different uses in my book.

I'm a backpack hater. When I have a backpack in people start treating me like a teenager. No thanks!