The guarantee for elite members is no different that the guarantee which I believe is offered by all hotel chains. If your confirmed reservation is not available you get a free night at another comparable hotel, a phone call home, transportation to the other hotel (and back if you are staying multiple nights). Not sure if this is a AMEX/Visa/MC requirement, just the practice which has been adopted, or whatever, but it is what is offered up when you insist.

The usual fight is over what is a "comparable" hotel.