I've got 3 cards at the moment. 2 have no balance on them, and I have no idea why I even still have them open. Main one that the Mrs and I use currently has a whopping £11.37 on it just now.
We use it for almost all our purchases, shopping, booking holidays etc. Once we use it in Morrisons for example (123 card for cashback) we'll make a payment to it using the app as soon as we get in the car. Gets us a wee bit of cashback every month, it all helps!
Regarding those with £15k on credit cards, I don't think I could sleep at night knowing that. Conversely, I bet nobody blinks an eye at having a £20k loan for a car though.