Dear Peter,
Thanks for your illuminating comments. It may be
necessary for me to give some more details of the case
at hand:
We're talking about an airline here. Transmission of
details is via https, so may be assumed secure. Also,
it is not possible to see your details if you log in
as yourself. However, it is possible to book
additional tickets on your credit card with arbitrary
passenger names - I thought some of the John Smithes
in this world might be interested in having a free
flight on my card. They would never be traceable
unless I check my credit card statement before they
board (or if we are going to assume the airline more
capable, leave) the plane, and alert the airline of
the fraud.
I've asked them to delete my details. They have not
informed me that they have done so, although they got
in touch once to claim that their site is perfectly
secure. I also out of habit chose an unguessable (i.e.
unmemorable) password, so I cannot now access my own
details unless I have them send my password
unencrypted. The username is just the email address,
so no security there, either. (Note to deviants: not
this email address.)