I have always presumed that the £3 deposit is charged to cover the potential
cost of somebody making a multi-zone journey when there isn't enough credit


I thought it was something to do with not wanting daytrippers getting
a card for one day only and walking off with it.

Isn't the point of oyster so that regular travellers (commuters) don't
need to get a new paper ticket each month / every time they sit on it?
If someone is only every going to buy a one day travelcard there is
no point issuing them with an expensive plastic card.