I have had friends hook me up with corporate rates, and since they usually have the company travel office make the booking for me there is no risk. As far as the hotel is concerned I am entitled to use the rate.
I would be cautious of using rate codes myself, but I really can't blame people for trying to save on their hotel bookings. Of course it is a morally questionable practice to fraudulently use corporate rates, but on the other hand it is arguably morally questionable for sub par large Hyatt city hotels to charge exorbitant rates (GH New York comes to mind).
I say do it at your own risk if you must, but be prepared to pay rack if you get caught. After all while the practice cannot be condoned, I am not sure hotels and companies do not benefit somewhat from a certain amount of misuse of corporate rates. If someone who would not otherwise have paid rack rate stays on a corporate rate, that is still revenue for the hotel. Likewise more people staying on a corporate rate contributes to the volume of business the hotel is (at least purportedly) getting from that company.