"Robin" wrote:
Why on earth would anyone who visits London 2 or 3 times a year have
any use for auto-top up?

Convenience?

I appreciate that this convenience comes at a cost. But let's quantify
the cost.

An occasional user is likely to opt to add GBP 20 every time the balance
on the card falls below GBP 10. So let's assume an average balance of
GBP 20.

We then have to look at the cost of that money. If the user is running
credit card debts on which the interest rate is 20 per cent the extra
borrowing of an average GBP 20 at 20 percent costs GBP 4 a year.
If OTOH the money would be in a savings account yielding 1.5 per cent
after tax the cost would be 30 pence a year.

I don't know how many users would see 30 pence or 4 pounds as a
reasonable price to pay to save the time/worry/risks of manual top-ups.
But I don't see it as out-of-the question. Eg if a user values their
own time at GBP 10 an hour those costs equate to 2 or 24 minutes.
Compared with logging on to top-up manually online, in shops or at
ticket machines............

The key thing for an occasional user is not having to nominate a gate line
for the top-up to be collected, nor having to queue for the busy ticket
machines at a major station. What's more, it can be activated on buses and
trams, unlike a normal online top-up.