As well as being useful for some travellers, if an auto-topup system
was introduced this would add to the potential appeal of Oyster as an
e-Money system, as it'd ensure that Oyster card holders never ran out
of credit when they want to buy a newspaper/pint of milk etc.

Personally, I think that this would lessen the appeal.
If you lost it you might just as well have handed someone
your house keys and said "take what you like"

tim