I agree with you that this check isn't fool/hacker proof. However it is useful as an additional check. Believe me I've been there and had to deal with the chargebacks ;-)

If you are talking about credit card transactions (which I was) then you are talking about secure transactions via https which in general aren't proxied.

Of course most anyone technical can hide their true IP address but in our case that wasn't the sort of person we were dealing with.