Cash has great privacy, works at cash only places, small merchants are happy to receive it because it means no fees, you get a discount when buying gas, the aforementioned tipping (good luck giving the valet a credit card!), it works when the power is out and a lot more I'm sure. Cash is pretty great. Even if you don't carry it you should definitely keep some at home.
Cash has great privacy - for some people, this is a non-issue. For me, I actually like the fact that credit cards have a record, at the end of the month I can look at my statement, and see all the places I spent money and how much. I can graph it, etc.
Works at cash only places - this probably varies around the world. In Australia, most places will accept card - the exception being Asian places, cause we're cheap =).
Small merchants are happy to receive it - I suspect this is changing, the fees are next to nothing these days, and by not handling cash, they avoid issues like employee theft, and also people are encouraged to spend more.
You get a discount when buying gas - we don't have this here (Australia)
The aforementioned tipping - likewise, we don't have this here. In general, I prefer to know how much I am spending beforehand, and I dislike the concept of tipping, or people needing to beg for tips.
It works when the power out - err, I suspect if the power is out in say a supermarket or a restaurant, they would have more issues than just the credit card machine not working.
I'm not saying never carry cash - but I like the convenience of card. And let's be honest most people aren't keeping their money stuffed inside their mattress.
Their money is in a bank account - and not in physical notes either, it's a set of electronic digits. So the whole, going to a bank, converting it to cash, giving it to somebody, so that they can then convert it back into electronic digits - seems incredibly archaic and outmoded.
Imagine that you're on a date. You've got a nice evening planned, only to run into a state-wide blackout. Now the trick is to get food at all. You see this restaurant with a lot of cars out front. Turns out they have gas grills, so they can cook some kinds of food. (Not french fries, though - that machine's electric). They're working by candlelight, adding up the tab on a hand calculator, pouring pop out of bottles, and they are not taking credit cards.
Been there, done that. Only once, but yeah, it happens.
The last time I had to go shopping during a blackout, the few places I went to that were open were still taking credit cards. Maybe it's just a regional thing.