I don't believe that is true. My understanding is that they *are* reimbursed, just not 100% of face value (in other words, maybe $80 credit for a $100 Bonus Buck).

And it shouldn't be applied as a "discount" (although some Front Desk folks try to do that). It is indeed a form of payment, and you should get Marriott Rewards points for the face amount (e.g., at a Full Service property, 10 points per dollar plus elite bonus). Marriott Concierge has confirmed that a number of times in this forum. If it's applied as a discount, you lose out on those points.