I have those, plus a debit card that I use for all day-to-day purchases. Apparently we're in the minority, though.

Amex primarily, Discover for places that don't take Amex (which is a lot more than you'd think), Visa debit (through an investment broker, so it has much higher daily limits) as a backup for the above.

It sounds silly, but over time you realize those edge cases that are a pain in the ass if you haven't planned ahead (I need $1000 accessible via debit immediately because a family member has lost their wallet across the country and I need to Western Union them some cash to get home).

No, I totally get it. I have my work Amex for travel, my personal debit for all personal daily spending, and a Visa for the case where one of those cards doesn't work. I don't want to be stranded in another country without money just because my debit card got compromised and blocked.

My wife enjoys playing with different rewards cards to take advantage of various temporary details. I often have one card for restaurants, one for groceries, one for normal purchases, plus an American Express for Costco.

I'm also a bit paranoid and have a fear of getting stuck somewhere because one of my cards stopped working, so I like having a large collection with me.

For me, the credit cards wouldn't be the most useful, but rather the membership cards like Costco, Zipcar, gym, etc. The problem is that none of these cards are standardized (Costco has a photo of the member, Zipcar is a prox card, gym card is scanned by barcode rather than magnetically, etc.)