Chip and run

I've had a number of customers give me their husband/wife's card - at least that's what they claim when their card says they're Nigel , when they're probably not!
On Monday night someone claimed that Chip and Pin is secure because it means that no one can use your card without your permission(!) - so if you know the pin that means you're a licit user. I refused the card but said they could use the local ATM machine and then pay for their order. They weren't happy - I said someone could see their number when they were using a cash machine and steal their card so it couldn't be totally secure - they weren't convinced. They went to get the money - 5 minutes went by - and they didn't return - obviously off in a huff - unless it wasn't their card?!
Luckily the other person waiting in the checkout was understanding.
How widespread is using your partner's card? - I'm seeing one or two cases a week when I'm working. To my mind it's just like forging your partners signature because you know they won't mind...