You're sharing pins for electronic transactions? What the what? (I'm over the pond in Australia, and I've never shared any of my various pins with anyone.)
I wouldn't! A lot of people do though, it's the downside of a mostly cashless economy, but with the rise of internet banking and near-instant transfers online it should go away.
As someone who knows someone else's PIN: if they're too inconvenienced by making the payment themselves and don't realize what security it's meant to offer, they'll be quite content to hand the card and the PIN to someone they trust.
