There may or may not be ethical issues using corp codes you're not supposed to use. In the case of Marriott, the codes are easy to guess, often give great rates and verification of eligibility is difficult.

I've had two stays in two different European Marriotts using such "non-authorized" corp rate, nobody even blinked, not to mention asking for any eligibility. It would be quite difficult to dis-prove that the co on my visit card is not an affiliate/supplier/contractor/consultant to the company whose rate I was staying on.

As the FT policy is not to discuss such corp rate codes, I do not go into details.