I am currently on a long term assignment, and the company I am consulting for has a policy that does not allow associates to have hotel stays or other travel discounts that are not for our own company. I think they do this for a few reasons -- namely, ethics, to assure we are not getting favors, and to make sure we get credit for hotel stays for negotiating in the future.
In this case -- even though I prefer Marriotts in many cases -- it would be pretty lame to get fired (contract pulled) for using another companies rates.
Sometimes there is more than just the rate check at the front desk to worry about.