For those of us who have power of attourney; having to use a PIN comes in very handy, both mum's card and my card are identical (save for the account number) ie. they both have my name on; so when I take it out of the wallet I forget which is which, but they both have different PIN numbers, so if I buy something for me it won't accept mum's PIN number (and vice-versa) which is handy!!
Anyhow that's an aside, I think you should be able to set your own limit, and have the power to block your card if you lose it or think you won't need it for a while; and that could be done so easily on-line or on your mobile, and I think some banks allow that anyway.
For security one day I'm sure we will be swiping our card AND inputting a fingerprint; as you aren't supposed to use someone else's card even with their permission.